The works of Francis Maitland Balfour, Volume 3 (of 4)

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Title: The works of Francis Maitland Balfour, Volume 3 (of 4)
A treatise on comparative embryology: Vertebrata

Author: Francis M. Balfour

Editor: Sir M. Foster
Adam Sedgwick

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Most recently updated: October 24, 2024

Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK THE WORKS OF FRANCIS MAITLAND BALFOUR, VOLUME 3 (OF 4) ***

THE WORKS
OF
FRANCIS MAITLAND BALFOUR.

VOL. III.

Memorial Edition.

Cambridge:

PRINTED BY C. J. CLAY, M.A. AND SON,
AT THE UNIVERSITY PRESS.

Memorial Edition.

THE WORKS

FRANCIS MAITLAND BALFOUR,

M.A., LL.D., F.R.S.,

FELLOW OF TRINITY COLLEGE,
AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY OF
CAMBRIDGE.

EDITED BY

M. FOSTER, F.R.S.,

PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE;

AND

ADAM SEDGWICK, M.A.,

FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE.

VOL. III.

A TREATISE ON COMPARATIVE EMBRYOLOGY.

Vol. II. Vertebrata.

London:
MACMILLAN AND CO.
1885

[_The Right of Translation is reserved._]

PREFACE TO VOLUME II.

The present volume completes my treatise on Comparative Embryology.
The first eleven chapters deal with the developmental history of the
Chordata. These are followed by three comparative chapters completing
the section of the work devoted to Systematic Embryology. The
remainder of the treatise, from Chapter XIV. onwards, is devoted to
Organogeny. For the reasons stated in the introduction to this part
the organogeny of the Chordata has been treated with much greater
fulness than that of the other groups of Metazoa.

My own investigations have covered the ground of the present volume
much more completely than they did that of the first volume; a not
inconsiderable proportion of the facts recorded having been directly
verified by me.

The very great labour of completing this volume has been much
lightened by the assistance I have received from my friends and
pupils. Had it not been for their co-operation a large number of the
disputed points, which I have been able to investigate during the
preparation of the work, must have been left untouched.

My special thanks are due to Mr Sedgwick, who has not only devoted a
very large amount of time and labour to correcting the proofs, but has
made for me an index of this volume, and has assisted me in many other
ways.

Dr Allen Thomson and Professor Kleinenberg of Messina have undertaken
the ungrateful task of looking through my proof-sheets, and have made
suggestions which have proved most valuable. To Professors Parker,
Turner, and Bridge, I am also greatly indebted for their suggestions
with reference to special chapters of the work.

CONTENTS OF VOLUME II.

CHAPTER I. CEPHALOCHORDA. Pp. 1-8.

Segmentation and formation of the layers, pp. 1-3. Central nervous
system, pp. 3, 4. Mesoblast, p. 5. General history of larva, pp. 6-8.

CHAPTER II. UROCHORDA. Pp. 9-39.

_Solitaria_, pp. 9-23. Development of embryo, pp. 9-15. Growth and
structure of free larva, pp. 15-19. Retrogressive metamorphosis, pp.
19-23. _Sedentaria_, p. 23. _Natantia_, pp. 23-28. _Doliolidæ_, pp.
28, 29. _Salpidæ_, pp. 29-34. _Appendicularia_, p. 34. Metagenesis,
pp. 34-38.

CHAPTER III. ELASMOBRANCHII. Pp. 40-67.

Segmentation and formation of the layers, pp. 40-47. Epiblast, p. 47.
Mesoblast, pp. 47-51. Hypoblast and notochord, pp. 51-54. General
features of the embryo at successive stages, pp. 55-62. The yolk-sack,
pp. 62-66.

CHAPTER IV. TELEOSTEI. Pp. 68-82.

Segmentation and formation of the layers, pp. 68-73. General history
of the layers, pp. 73-75. General development of the embryo, pp.
76-81.

CHAPTER V. CYCLOSTOMATA. Pp. 83-101.

Segmentation and formation of the layers, pp. 83-86. Mesoblast and
notochord, pp. 86, 87. General history of the development, pp. 87-97.
Metamorphosis, pp. 97-100. Myxine, p. 100.

CHAPTER VI. GANOIDEI. Pp. 102-119.

_Acipenser_, pp. 102-110. Segmentation and formation of the layers,
pp. 102-104. General development of the embryo and larva, pp. 104-110.
_Lepidosteus_, pp. 111-119. Segmentation, pp. 111, 112. General
development of embryo and larva, pp. 112-119. General observations on
the embryology of Ganoids, p. 119.

CHAPTER VII. AMPHIBIA. Pp. 120-144.

Oviposition and impregnation, pp. 120, 121. Formation of the layers,
pp. 121-124. Epiblast, pp. 125-127. Mesoblast and notochord, pp. 128,
129. Hypoblast, pp. 129-131. _General growth of the embryo_, pp.
131-143. Anura, pp. 131-141. Urodela, pp. 141-143. Gymnophiona, p.
143.

CHAPTER VIII. AVES. Pp. 145-201.

Segmentation and formation of the layers, pp. 145-166. General history
of the germinal layers, pp. 166-169. General development of the
embryo, pp. 169-180. _Foetal membranes_, pp. 185-199. Amnion, pp.
185-191. Allantois, pp. 191-193. Yolk-sack, pp. 193-199.

CHAPTER IX. REPTILIA. Pp. 202-213.

_Lacertilia_, pp. 202-209. Segmentation and formation of the layers,
pp. 202-207. General development of the embryo, p. 208. Embryonic
membranes and yolk-sack, pp. 208-210. _Ophidia_, p. 210. _Chelonia_,
pp. 210-212.

CHAPTER X. MAMMALIA. Pp. 214-274.

Segmentation and formation of the layers, pp. 214-227. General growth
of the embryo, pp. 227-232. Embryonic membranes and yolk-sack, pp.
232-239. Comparative history of the Mammalian foetal membranes, pp.
239-257. Comparative histology of the placenta, pp. 257-259. Evolution
of the placenta, pp. 259-261. Development of the Guinea-pig, pp.
262-265. The human embryo, pp. 265-270.

CHAPTER XI. COMPARISON OF THE FORMATION OF THE GERMINAL LAYERS AND OF
THE EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES. Pp. 275-310.

Formation of the gastrula, pp. 275-292. The formation of the mesoblast
and of the notochord, pp. 292-300. _The epiblast_, pp. 300-304.
Formation of the central nervous system, pp. 301-304. Formation of the
organs of special sense, p. 304. Summary of organs derived from the
three germinal layers, pp. 304-306. Growth in length of the Vertebrate
embryo, pp. 306-309. The evolution of the allantois and amnion, pp.
309, 310.

CHAPTER XII. OBSERVATIONS ON THE ANCESTRAL FORM OF THE CHORDATA. Pp.
311-330.

General considerations, pp. 311-316. The medullary canal, pp. 316,
317. The origin and nature of the mouth, pp. 317-321. The cranial
flexure, pp. 321, 322. The postanal gut and neurenteric canal, pp.
322-325. The body-cavity and mesoblastic somites, p. 325. The
notochord, pp. 325, 326. Gill clefts, pp. 326, 327. Phylogeny of the
Chordata, pp. 327-329.

CHAPTER XIII. GENERAL CONCLUSIONS. Pp. 331-388.

I. Mode of origin and homologies of the germinal layers, pp. 331-360.
Formation of the primary germinal layers, pp. 332, 333. Invagination,
pp. 333-335. Delamination, pp. 335-338. Phylogenetic significance of
delamination and invagination, pp. 338-345. Homologies of the germinal
layers, pp. 345, 346. The origin of the mesoblast, pp. 346-360.

II. Larval forms: their nature, origin, and affinities. Preliminary
considerations, pp. 360-362. Types of larvæ, pp. 363-384. Phylogenetic
conclusions, pp. 384, 385. General conclusions and summary, pp. 385,
386.

PART II. ORGANOGENY;

INTRODUCTION. Pp. 391, 392.

CHAPTER XIV. THE EPIDERMIS AND ITS DERIVATIVES. Pp. 393-399.

Protective epidermic structures, pp. 393-397. Dermal skeletal
structures, p. 397. Glands, pp. 397, 398.

CHAPTER XV. THE NERVOUS SYSTEM. Pp. 400-469.

The origin of the nervous system, pp. 400-405. Nervous system of the
Invertebrata, pp. 405-414. _Central nervous system of the Vertebrata_,
pp. 415-447. Spinal chord, pp. 415-419. General development of the
brain, pp. 419-423. Hind-brain, pp. 424-427. Mid-brain, pp. 427, 428.
General development of fore-brain, pp. 428-430. Thalamencephalon, pp.
430-435. Pituitary body, pp. 435-437. Cerebral Hemispheres, pp.
437-444. Olfactory lobes, pp. 444, 445. General conclusions as to the
central nervous system of the Vertebrata, pp. 445-447. _Development of
the cranial and spinal nerves_, pp. 448-466. Spinal nerves, pp.
448-455. Cranial nerves, pp. 455-466. _Sympathetic nervous system_,
pp. 466-468.

CHAPTER XVI. ORGANS OF VISION. Pp. 470-511.

Coelenterata, pp. 471, 472. Mollusca, pp. 472-479. Chætopoda, p. 479.
Chætognatha, p. 479. Arthropoda, pp. 479-483. Vertebrata general, pp.
483-490. Retina, pp. 490-492. Optic nerve, pp. 492, 493. Choroid
fissure, p. 493. Lens, pp. 494, 495. Vitreous humour, pp. 494, 495.
Cornea, pp. 495-497. Aqueous humour, p. 497. _Comparative development
of Vertebrate eye_, pp. 497-506. Ammocoete eye, pp. 498, 499. Optic
vesicles, p. 499. Lens, p. 499. Cornea, p. 500. Optic nerve and
choroid fissure, pp. 500-505. Iris and ciliary processes, p. 506.
Accessory organs connected with the eye, p. 506. Eyelids, p. 506.
Lacrymal glands, p. 506. Lacrymal duct, pp. 506, 507. _Eye of the
Tunicata_, pp. 507-509. Accessory eyes in the Vertebrata, pp. 509,
510.

CHAPTER XVII. AUDITORY ORGAN, OLFACTORY ORGAN, AND SENSE ORGANS OF THE
LATERAL LINE. Pp. 512-541.

Auditory organs, pp. 512-531. General structure of auditory organs,
pp. 512, 513. Auditory organs of the Coelenterata, pp. 513-515.
Auditory organs of the Mollusca, pp. 515, 516. Auditory organs of the
Crustacea, p. 516. _Auditory organs of the Vertebrata_, pp. 516-530.
Auditory vesicle, pp. 517-524. Organ of Corti, pp. 524-527. Accessory
structures connected with the organ of hearing of terrestrial
vertebrata, pp. 527-530. Auditory organ of the Tunicata, pp. 530, 531.
Bibliography of Auditory organs, p. 531.

Olfactory organs, pp. 531-538. Bibliography of Olfactory organs, p.
538. Sense organs of the lateral line, pp. 538-540. Bibliography of
sense organs of lateral line, pp. 540, 541.

CHAPTER XVIII. THE NOTOCHORD, THE VERTEBRAL COLUMN, THE RIBS, AND THE
STERNUM. Pp. 542-563.

Introductory remarks on the origin of the skeleton, pp. 542-544.
Bibliography of the origin of the skeleton, pp. 544, 545. The
notochord and its cartilaginous sheath, pp. 545-549. _The vertebral
arches and the vertebral bodies_, pp. 549-559. Cyclostomata, p. 549.
Elasmobranchii, pp. 549-553. Ganoidei, p. 553. Teleostei, p. 553.
Amphibia, pp. 553-556. Reptilia, pp. 556, 557. Aves, pp. 557, 558.
Mammalia, pp. 558, 559. Bibliography of the notochord and vertebral
column, p. 560. _Ribs_, pp. 560-562. _Sternum_, pp. 562, 563.
Bibliography of the ribs and sternum, p. 563.

CHAPTER XIX. THE SKULL. Pp. 564-598.

Preliminary remarks, pp. 564, 565. The cartilaginous cranium, pp.
565-571. The parachordals and notochord, pp. 566, 567. The trabeculæ,
pp. 567-570. The sense capsules, pp. 570, 571. The branchial skeleton,
pp. 572-591. General structure of, pp. 572-575. _Mandibular and hyoid
arches_, pp. 575-591. Elasmobranchii, pp. 576-579. Teleostei, pp.
579-581. Amphibia, pp. 581-588. Sauropsida, pp. 588, 589. Mammalia,
pp. 589-591. Membrane bones and ossifications of the cranium, pp.
592-597. Membrane bones, pp. 592-595. Ossifications of the
cartilaginous cranium, pp. 595-597. Labial cartilages, p. 597.
Bibliography of the skull, p. 598.

CHAPTER XX. PECTORAL AND PELVIC GIRDLES AND THE SKELETON OF THE LIMBS.
Pp. 599-622.

The Pectoral girdle, pp. 599-606. Pisces, pp. 599-601. _Amphibia and
Amniota_, pp. 601, 602. Lacertilia, p. 603. Chelonia, p. 603. Aves,
pp. 603, 604. Mammalia, p. 604. Amphibia, p. 605. Bibliography of
Pectoral girdle, pp. 605, 606.

The Pelvic girdle, pp. 606-608. Pisces, pp. 606, 607. _Amphibia and
Amniota_, pp. 606, 607. Amphibia, p. 607. Lacertilia, p. 607.
Mammalia, p. 608. Bibliography of Pelvic girdle, p. 608. _Comparison
of pectoral and pelvic girdles_, pp. 608, 609.

Limbs, pp. 609-622. The piscine fin, pp. 609-618. The cheiropterygium,
pp. 618-622. Bibliography of limbs, p. 622.

CHAPTER XXI. THE BODY CAVITY, THE VASCULAR SYSTEM AND THE VASCULAR
GLANDS. Pp. 623-666.

The body cavity, pp. 623-632. General, pp. 623, 624. _Chordata_, pp.
624-632. Abdominal pores, pp. 626, 627. Pericardial cavities, pleural
cavities and diaphragm, pp. 627-632. Bibliography of body cavity, p.
632.

The vascular system, pp. 632-663. General, pp. 632, 633. The heart,
pp. 633-643. Bibliography of the heart, p. 643. Arterial system, pp.
643-651. Bibliography of the arterial system, p. 651. Venous system,
pp. 651-663. Bibliography of the venous system, p. 663. Lymphatic
system and spleen, p. 664. Bibliography of spleen, p. 664. Suprarenal
bodies, pp. 664-666. Bibliography of suprarenal bodies, p. 666.

CHAPTER XXII. THE MUSCULAR SYSTEM. Pp. 667-679.

Evolution of muscle-cells, pp. 667, 668. _Voluntary muscular system of
the Chordata_, pp. 668-679. Muscular fibres, pp. 668, 669. Muscular
system of the trunk and limbs, pp. 673-676. The somites and muscular
system of the head, pp. 676-679. Bibliography of muscular system, p.
679.

CHAPTER XXIII. EXCRETORY ORGANS. Pp. 680-740.

Platyelminthes, pp. 680, 681. Mollusca, pp. 681, 682. Polyzoa, pp.
682, 683. Branchiopoda, p. 683. Chætopoda, pp. 683-686. Gephyrea, pp.
686, 687. Discophora, pp. 687, 688. Arthropoda, pp. 688, 689.
Nematoda, p. 689. Excretory organs and generative ducts of the
Craniata, pp. 689-737. General, pp. 689, 690. Elasmobranchii, pp.
690-699. Cyclostomata, pp. 700, 701. Teleostei, pp. 701-704. Ganoidei,
pp. 704-707. Dipnoi, p. 707. Amphibia, pp. 707-713. Amniota, pp.
713-727. General conclusions and summary, pp. 728-737. Pronephros, pp.
728, 729. Mesonephros, pp. 729-732. Genital ducts, pp. 732-736.
Metanephros, pp. 736, 737. _Comparison of the excretory organs of the
Chordata and Invertebrata_, pp. 737, 738. Bibliography of Excretory
organs, pp. 738-740.

CHAPTER XXIV. GENERATIVE ORGANS AND GENITAL DUCTS. Pp. 741-753.

Generative organs, pp. 741-748. Porifera, p. 741. Coelenterata, pp.
741-743. Chætopoda and Gephyrea, p. 743. Chætognatha, pp. 743-745.
Polyzoa, p. 745 Nematoda, p. 745. Insecta, p. 745. Crustacea, pp. 745,
746. Chordata, pp. 746-748. Bibliography of generative organs, p. 748.
Genital ducts, pp. 748-753.

CHAPTER XXV. THE ALIMENTARY CANAL AND ITS APPENDAGES IN THE CHORDATA.
Pp. 754-780.

Mesenteron, pp. 754-774. Subnotochordal rod, pp. 754-756. Splanchnic
mesoblast and mesentery, pp. 756-758. _Respiratory division of the
Mesenteron_, pp. 758-766. Thyroid body, pp. 759-762. Thymus gland, pp.
762, 763. Swimming bladder and lungs, pp. 763-766, _The middle
division of the Mesenteron_, pp. 766-771. Cloaca, pp. 766, 767.
Intestine, pp. 767, 768. Liver, pp. 769, 770. Pancreas, pp. 770, 771.
_Postanal section of the Mesenteron_, pp. 771-774.

The stomodæum, pp. 774-778. Comparative development of oral cavity,
pp. 774-776. Teeth, pp. 776-778.

The proctodæum, pp. 778-780. Bibliography of alimentary canal, p. 780.

EMBRYOLOGY.

CHAPTER I.

CEPHALOCHORDA.

The developmental history of the Chordata has been studied far more
completely than that of any of the groups so far considered; and the
results which have been arrived at are of striking interest and
importance. Three main subdivisions of this group can be recognized:
(1) the Cephalochorda containing the single genus Amphioxus; (2) the
Urochorda or Tunicata; and (3) the Vertebrata[1]. The members of the
second and probably of the first of these groups have undergone
degeneration, but at the same time the members of the first group
especially undergo a less modified development than that of other
Chordata.

[1] The term Vertebrata is often used to include the
Cephalochorda. It is in many ways convenient to restrict its use
to the forms which have at any rate some indications of vertebræ;
a restriction which has the further convenience of restoring to
the term its original limitations. In the first volume of this
work the term Craniata was used for the forms which I now propose
to call Vertebrata.

CEPHALOCHORDA.

Our knowledge of the development of Amphioxus is mainly due to
Kowalevsky (Nos. 1 and 2). The ripe eggs appear to be dehisced into
the branchial or atrial cavity, and to be transported thence through
the branchial clefts into the pharynx, and so through the mouth to the
exterior. (Kowalevsky, No. 1, and Marshall, No. 5.)

[FIG. 1. THE SEGMENTATION OF AMPHIOXUS. (Copied from Kowalevsky.)

B. Stage with four equal segments.
C. Stage after the four segments have become divided by an
equatorial furrow into eight equal segments.
D. Stage in which a single layer of cells encloses a central
segmentation cavity.
E. Somewhat older stage in optical section.

_sg._ segmentation cavity.]

When laid the egg is about 0.105 mm. in diameter. It is invested by a
delicate membrane, and is somewhat opaque owing to the presence of
yolk granules, which are however uniformly distributed through it, and
proportionately less numerous than in the ova of most Chordata.
Impregnation is external and the segmentation is nearly regular (fig.
1). A small segmentation cavity is visible at the stage with four
segments, and increases during the remainder of the segmentation; till
at the close (fig. 1 E) the embryo consists of a blastosphere formed
of a single layer of cells enclosing a large segmentation cavity. One
side of the blastosphere next becomes invaginated, and during the
process the embryo becomes ciliated, and commences to rotate. The
cells forming the invaginated layer become gradually more columnar
than the remaining cells, and constitute the hypoblast; and a
structural distinction between the epiblast and hypoblast is thus
established. In the course of the invagination the segmentation cavity
becomes gradually obliterated, and the embryo first assumes a
cup-shaped form with a wide blastopore, but soon becomes elongated,
while the communication of the archenteron, or cavity of invagination,
with the exterior is reduced to a small blastopore (fig. 2 A), placed
at the pole of the long axis which the subsequent development shews to
be _the hinder end of the embryo_. The blastopore is often known in
other Chordata as the anus of Rusconi. Before the invagination is
completed the larva throws off the egg-membrane, and commences to lead
a free existence.

[FIG. 2. EMBRYOS OF AMPHIOXUS. (After Kowalevsky.)

The parts in black with white lines are epiblastic; the shaded parts
are hypoblastic.
A. Gastrula stage in optical section.
B. Slightly later stage after the neural plate _np_ has become
differentiated, seen as a transparent object from the dorsal side.
C. Lateral view of a slightly older larva in optical section.
D. Dorsal view of an older larva with the neural canal completely
closed except for a small pore (_no_) in front.
E. Older larva seen as a transparent object from the side.

_bl._ blastopore (which becomes in D the neurenteric canal); _ne._
neurenteric canal; _np._ neural or medullary plate; _no._ anterior
opening of neural canal; _ch._ notochord; _so^I_, _so^{II}._ first
and second mesoblastic somites.]

Up to this stage the larva, although it has acquired a cylindrical
elongated form, has only the structure of a simple two-layered
gastrula; but the changes which next take place give rise on the one
hand to the formation of the central nervous system, and on the other
to the formation of the notochord and mesoblastic somites[2]. The
former structure is developed from the epiblast and the two latter
from the hypoblast.

[2] The protovertebræ of most embryologists will be spoken of as
mesoblastic somites.

The formation of the central nervous system commences with the
flattening of the dorsal surface of the embryo. The flattened area
forms a plate (fig. 2 B and fig. 3 A, _np_), extending backwards to
the blastopore, which has in the meantime passed round to the dorsal
surface. The sides of the plate become raised as two folds, which are
most prominent posteriorly, and meet behind the blastopore, but shade
off in front. The two folds next unite dorsally, so as to convert the
previous groove into a canal[3]--the neural or medullary canal. They
unite first of all over the blastopore, and their line of junction
extends from this point forwards (fig. 2 C, D, E). There is in this
way formed a tube on the floor of which the blastopore opens behind,
and which is itself open in front. Finally the medullary canal is
formed for the whole length of the embryo. The anterior opening
persists however for some time. The communication between the neural
and alimentary tracts becomes interrupted when the caudal fin appears
and the anus is formed. The neural canal then extends round the end of
the notochord to the ventral side, but subsequently retreats to the
dorsal side and terminates in a slight dilatation.

[3] The details of this process are spoken of below.

In the formation of the medullary canal there are two points deserving
notice--viz. (1) the connection with the blastopore; (2) the relation
of the walls of the canal to the adjoining epiblast. With reference to
the first of these points it is clear that the fact of the blastopore
opening on the floor of the neural canal causes a free communication
to exist between the archenteron or gastrula cavity and the neural
canal; and that, so long as the anterior pore of the neural canal
remains open, the archenteron communicates indirectly with the
exterior (_vide_ fig. 2 E). It must not however be supposed (as has
been done by some embryologists) that the pore at the front end of the
neural canal represents the blastopore carried forwards. It is even
probable that what Kowalevsky describes as the carrying of the
blastopore to the dorsal side is really the commencement of the
formation of the neural canal, the walls of which are continuous with
the lips of the blastopore. This interpretation receives support from
the fact that at a later stage, when the neural and alimentary canals
become separated, the neural canal extends round the posterior end of
the notochord to the ventral side. The embryonic communication between
the neural and alimentary canals is common to most Chordata; and the
tube connecting them will be called the neurenteric canal. It is
always formed in fundamentally the same manner as in Amphioxus. With
reference to the second point it is to be noted that Amphioxus is
exceptional amongst the Chordata in the fact that, before the closure
of the neural groove, the layer of cells which will form the neural
tube becomes completely separated from the adjoining epiblast (fig. 3
A), and forms a structure which may be spoken of as the medullary
plate; and that in the closure of the neural canal the lateral
epiblast forms a complete layer above this plate before the plate
itself is folded over into a closed canal. This peculiarity will be
easily understood from an examination of fig. 3 A, B and C.

[FIG. 3. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES.
(After Kowalevsky.)

A. Section at gastrula stage.
B. Section of an embryo slightly younger than that represented in
fig. 2 D.
C. Section through the anterior part of an embryo at the stage
represented in fig. 2 E.

_np._ neural plate; _nc._ neural canal; _mes._ archenteron in A and
B, and mesenteron in C; _ch._ notochord; _so._ mesoblastic somite.]

The formation of the mesoblastic somites commences, at about the same
time as that of the neural canal, as a pair of hollow outgrowths of
the walls of the archenteron. These outgrowths, which are shewn in
surface view in fig. 2 B and D, _so_, and in section in fig. 3 B and
C, _so_, arise near the front end of the body and gradually extend
backwards as wing-like diverticula of the archenteric cavity. As they
grow backwards their dorsal part becomes divided by transverse
constrictions into cubical bodies (fig. 2 D and E), which, with the
exception of the foremost, soon cease to open into what may now be
called the mesenteron, and form the mesoblastic somites. Each
mesoblastic somite, after its separation from the mesenteron, is
constituted of two layers, an inner one--the splanchnic--and an
outer--the somatic, and a cavity between the two which was originally
continuous with the cavity of the mesenteron. Eventually the dorsal
parts of the outgrowths become separated from the ventral, and form
the muscle-plates, while their cavities atrophy. The cavity of the
ventral part, which is not divided into separate sections by the above
described constrictions, remains as the true body cavity. The ventral
part of the inner layer of the mesoblastic outgrowths gives rise to
the muscular and connective tissue layers of the alimentary tract, and
the dorsal part to a section of the voluntary muscular system. The
ventral part of the outer layer gives rise to the somatic mesoblast,
and the dorsal to a section of the voluntary muscular system. The
anterior mesoblastic somite long retains its communication with the
mesenteron, and was described by Max Schultze, and also at first by
Kowalevsky, as a glandular organ. While the mesoblastic somites are
becoming formed the dorsal wall of the mesenteron develops a median
longitudinal fold (fig. 3 B, _ch_), which is gradually separated off
from before backwards as a rod (fig. 3 C, _ch_), underlying the
central nervous system. This rod is the notochord. After the
separation of those parts the remainder of the hypoblast forms the
wall of the mesenteron.

With the formation of the central nervous system, the mesoblastic
somites, the notochord, and the alimentary tract the main systems of
organs are established, and it merely remains briefly to describe the
general changes of form which accompany the growth of the larva into
the adult. By the time the larva is but twenty-four hours old there
are formed about seventeen mesoblastic somites. The body, during the
period in which these are being formed, remains cylindrical, but
shortly afterwards it becomes pointed at both ends, and the caudal fin
appears. The fine cilia covering the larva also become replaced by
long cilia, one to each cell. The mesenteron is still completely
closed, but on the right side of the body, at the level of the front
end of the mesenteron, the hypoblast and epiblast now grow together,
and a perforation becomes formed through their point of contact, which
becomes the mouth. The anus is probably formed about the same time if
not somewhat earlier[4].

[4] The lateral position of the mouth in the embryo Amphioxus has been
regarded as proving that the mouth represents a branchial cleft,
but the general asymmetry of the organs is such that no great
stress can, I think, be laid on the position of the mouth.

[FIG. 4. SECTIONS THROUGH TWO ADVANCED EMBRYOS OF
AMPHIOXUS TO SHEW THE FORMATION OF THE PERIBRANCHIAL CAVITY. (After
Kowalevsky.)

In A are seen two folds of the body wall with a prolongation of the
body cavity. In B the two folds have coalesced ventrally, forming a
cavity into which a branchial cleft is seen to open.

_mes._ mesenteron; _br.c._ branchial cavity; _pp._ body cavity.]

Of the subsequent changes the two most important are (1) the formation
of the gill slits or clefts; (2) the formation of the peribranchial or
atrial cavity.

The formation of the gill slits is, according to Kowalevsky's
description, so peculiar that one is almost tempted to suppose that
his observations were made on pathological specimens. The following is
his account of the process. Shortly after the formation of the mouth
there appears on the ventral line a coalescence between the epiblast
and hypoblast. Here an opening is formed, and a visceral cleft is thus
established, which passes to the left side, viz. the side opposite the
mouth. A second and apparently a third slit are formed in the same
way. The stages immediately following were not observed, but in the
next stage twelve slits were present, no longer however on the left
side, but in the median ventral line. There now appears on the side
opposite the mouth, and the same therefore as that originally occupied
by the first three clefts, a series of fresh clefts, which in their
growth push the original clefts over to the same side as the mouth.
Each of the fresh clefts becomes divided into two, which form the
permanent clefts of their side.

The gill slits at first open freely to the exterior, but during their
formation two lateral folds of the body wall, containing a
prolongation of the body cavity, make their appearance (fig. 4 A), and
grow downwards over the gill clefts, and finally meet and coalesce
along the ventral line, leaving a widish cavity between themselves and
the body wall. Into this cavity, which is lined by epiblast, the gill
clefts open (fig. 4 B, _br.c_). This cavity--which forms a true
peribranchial cavity--is completely closed in front, but owing to the
folds not uniting completely behind it remains in communication with
the exterior by an opening known as the atrial or abdominal pore.

The vascular system of Amphioxus appears at about the same time as the
first visceral clefts.

BIBLIOGRAPHY.

(1) A. Kowalevsky. "Entwicklungsgeschichte des Amphioxus lanceolatus."
_Mém. Acad. Impér. des Sciences de St Pétersbourg_, Series VII. Tom.
xi. 1867.

(2) A. Kowalevsky. "Weitere Studien über die Entwicklungsgeschichte
des Amphioxus lanceolatus." _Archiv f. mikr. Anat._, Vol. XIII. 1877.

(3) Leuckart u. Pagenstecher. "Untersuchungen über niedere Seethiere."
_Müller's Archiv_, 1858.

(4) Max Schultze. "Beobachtung junger Exemplare von Amphioxus." _Zeit.
f. wiss. Zool._, Bd. III. 1851.

(5) A. M. Marshall. "On the mode of Oviposition of Amphioxus." _Jour.
of Anat. and Phys._, Vol. X. 1876.

CHAPTER II.

UROCHORDA[5].

[5] The following classification of the Urochorda is adopted in
the present chapter.

I. Caducichordata.
{Solitaria _ex. Ascidia_.
A. SIMPLICIA {Socialia _ex. Clavellina_.
{Sedentaria _ex. Botryllus_.
B. COMPOSITA {Natantia _ex. Pyrosoma_.
{Salpidæ.
C. CONSERTA {Doliolidæ.
II. Perennichordata.
_Ex. Appendicularia_.

In the Solitaria, except Cynthia, the eggs are generally laid, and
impregnation is effected sometimes before and sometimes after the eggs
have left the atrial cavity. In Cynthia and most Caducichordata
development takes place within the body of the parent, and in the
Salpidæ a vascular connection is established between the parent and
the single foetus, forming a structure physiologically comparable with
the Mammalian placenta.

Solitaria. The development of the Solitary Ascidians has been more
fully studied than that of the other groups, and appears moreover to
be the least modified. It has been to a great extent elucidated by the
splendid researches of Kowalevsky (Nos. 18 and 20), whose statements
have been in the main followed in the account below. Their truth seems
to me to be established, in spite of the scepticism they have met with
in some quarters, by the closeness of their correspondence with the
developmental phenomena in Amphioxus.

The type most fully investigated by Kowalevsky is Ascidia (Phallusia)
mammillata; and the following description must be taken as more
especially applying to this type.

The segmentation is complete and regular. A small segmentation cavity
appears fairly early, and is surrounded, according to Kowalevsky, by a
single layer of cells, though on this point Kupffer (No. 27) and Giard
(No. 11) are at variance with him.

[FIG. 5. TRANSVERSE SECTION THROUGH THE FRONT END OF AN EMBRYO OF
PHALLUSIA MAMMILLATA. (After Kowalevsky.)

The embryo is slightly younger than that represented in fig. 8 III.

_mg._ medullary groove; _al._ alimentary tract.]

The segmentation is followed by an invagination of nearly the same
character as in Amphioxus. The blastosphere resulting from the
segmentation first becomes flattened on one side, and the cells on the
flatter side become more columnar (fig. 8 I.). Very shortly a
cup-shaped form is assumed, the concavity of which is lined by the
more columnar cells. The mouth of the cup or blastopore next becomes
narrowed; while at the same time the embryo becomes oval. The
blastopore is situated not quite at a pole of the oval but in a
position which subsequent development shews to _be on the dorsal side
close to the posterior end of the embryo_. The long axis of the oval
corresponds with the long axis of the embryo. At this stage the embryo
consists of two layers; a columnar hypoblast lining the central cavity
or archenteron, and a thinner epiblastic layer. The dorsal side of the
embryo next becomes flattened (fig. 8 II.), and the epiblast covering
it is shortly afterwards marked by an axial groove continued forwards
from the blastopore to near the front end of the body (fig. 5, _mg_).
This is the medullary groove, and it soon becomes converted into a
closed canal--the medullary or neural canal--below the external skin
(fig. 6, _n.c_). The closure is effected by the folds on each side of
the furrow meeting and coalescing dorsally. The original medullary
folds fall into one another behind the blastopore so that the
blastopore is situated on the floor of the groove, and, on the
conversion of the groove into a canal, the blastopore connects the
canal with the archenteric cavity, and forms a short neurenteric
canal. The closure of the medullary canal commences at the blastopore
and is thence continued forwards, the anterior end of the canal
remaining open. The above processes are represented in longitudinal
section in fig. 8 III, _n_. When the neural canal is completed for its
whole length, it still communicates by a terminal pore with the
exterior. In the relation of the medullary canal to the blastopore, as
well as in the closure of the medullary groove from behind forwards,
the Solitary Ascidians agree closely with Amphioxus.

[FIG. 6. TRANSVERSE OPTICAL SECTION OF THE TAIL OF AN EMBRYO OF
PHALLUSIA MAMMILLATA. (After Kowalevsky.)

The section is from an embryo of the same age as fig. 8 IV.

_ch._ notochord; _n.c._ neural canal; _me._ mesoblast; _al._
hypoblast of tail.]

The cells of the dorsal wall of the archenteron immediately adjoining
the front and sides of the blastopore have in the meantime assumed a
somewhat different character from the remaining cells of the
archenteron, and give rise to a body which, when viewed from the
dorsal surface, has somewhat the form of a horseshoe. This body was
first observed by Metschnikoff. On the elongation of the embryo and
the narrowing of the blastopore the cells forming this body arrange
themselves as a broad linear cord, two cells wide, underlying about
the posterior half of the neural canal (fig. 7, _ch_). They form the
rudiment of the notochord, which, as in Amphioxus, is derived from the
dorsal wall of the archenteron. They are seen in longitudinal section
in fig. 8 II. and III. _ch_.

With the formation of the notochord the body of the embryo becomes
divided into two distinct regions--a posterior region where the
notochord is present, and an anterior region into which it is not
prolonged. These two regions correspond with the tail and the trunk of
the embryo at a slightly later stage. The section of the archenteric
cavity in the trunk dilates and constitutes the permanent mesenteron
(figs. 7, _al_, and 8 III. and IV. _dd_). It soon becomes shut off
from the slit-like posterior part of the archenteron. The nervous
system in this part also dilates and forms what may be called the
cephalic swelling (fig. 8 IV.), and the pore at its anterior extremity
gradually narrows and finally disappears. In the region of the tail we
have seen that the dorsal wall of the archenteron becomes converted
into the notochord, which immediately underlies the posterior part of
the medullary canal, and soon becomes an elongated cord formed of a
single or double row of flattened cells. The lateral walls of the
archenteron (fig. 7, _me_) in the tail become converted into elongated
cells arranged longitudinally, which form powerful lateral muscles
(fig. 8 IV. _m_). After the formation of the notochord and of the
lateral muscles there remains of the archenteron in the tail only the
ventral wall, which according to Kowalevsky forms a simple cord of
cells (fig. 6, _al_). It is however not always present, or else has
escaped the attention of other observers. It is stated by Kowalevsky
to be eventually transformed into blood corpuscles. The neurenteric
canal leads at first into the narrow space between the above
structures, which is the remnant of the posterior part of the lumen of
the archenteron. Soon both the neurenteric canal and the caudal
remnant of the archenteron become obliterated.

[FIG. 7. OPTICAL SECTION OF AN EMBRYO OF PHALLUSIA MAMMILLATA.
(After Kowalevsky.)

The embryo is of the same age as fig. 8 III, but is seen in
longitudinal horizontal section.

part of body; _ch._ notochord; _me._ mesoblast.]

During the above changes the tail becomes considerably elongated and,
owing to the larva being still in the egg-shell, is bent over to the
ventral side of the trunk.

[FIG. 8. VARIOUS STAGES IN THE DEVELOPMENT OF PHALLUSIA MAMMILLATA.
(From Huxley; after Kowalevsky.)

The embryos are represented in longitudinal vertical section.
I. Commencing gastrula stage. _fh._ segmentation cavity.
II. Late gastrula stage with flattened dorsal surface. _eo._
blastopore; _ch._ notochord; _dd._ hypoblast.
III. A more advanced embryo with a partially-formed neural tube.
_ch._ and _dd._ as before; _n._ neural tube; _c._ epiblast.
IV. Older embryo in which the formation of the neural tube is
completed. _dd._ hypoblast enclosing persistent section of
alimentary tract; _dd´._ hypoblast in the tail; _m._ muscles.
V. Larva just hatched. The end of the tail is not represented. _a._
eye; _gb._ dilated extremity of neural tube with otolith projecting
into it; _Rg._ anterior swelling of the spinal division of the
neural tube; _f_. anterior pore of neural tube; _Rm._ posterior part
of neural tube; _o._ mouth; _Chs._ notochord; _kl._ atrial
invagination; _dd._ branchial region of alimentary tract; d.
commencement of oesophagus and stomach; _dd´._ hypoblast in the
tail; _m._ muscles; _hp._ papilla for attachment.
VI. Body and anterior part of the tail of a two days' larva. _klm._
atrial aperture; en. endostyle; _ks._ branchial sack; _1ks. 2ks._
branchial slits; _bb._ branchial vessel between them; _ch._ axial
portion of notochord; _chs._ peripheral layer of cells. Other
reference letters as before.]

The larva at this stage is represented in a side view in fig. 8 IV.
The epidermis is formed throughout of a single layer of cells. In the
trunk the mesenteron is shewn at _dd_ and the dilated part of the
nervous system, no longer communicating with the exterior, at _n_. In
the tail the notochord is shewn at _ch_, the muscles at _m_, and the
solid remnant of the ventral wall of the archenteron at _dd´_. The
delicate continuation of the neural canal in the tail is seen above
the notochord at _n_. An optical section of the tail is shewn in fig.
6. It is worthy of notice that the notochord and muscles are formed in
the same manner as in Amphioxus, except that the process is somewhat
simplified. The mode of disappearance of the archenteric cavity in the
tail, by the employment of the whole of its walls in the formation of
various organs, is so peculiar, that I feel some hesitation in
accepting Kowalevsky's statements on this head[6].

[6] It is more probable that this part of the alimentary tract is
equivalent to the postanal gut of many Vertebrata, which is at
first a complete tube, but disappears later by the simple
absorption of the walls.

The larva continues to grow in length, and the tail becomes further
curled round the ventral side of the body within the egg-membrane.
Before the tail has nearly reached its full length the test becomes
formed as a cuticular deposit of the epiblast cells (O. Hertwig, No.
13, Semper, No. 37). It appears first in the tail and gradually
extends till it forms a complete investment round both tail and trunk,
and is at first totally devoid of cells. Shortly after the
establishment of the test there grow out from the anterior end of the
body three peculiar papillæ, developed as simple thickenings of the
epidermis. At a later stage, after the hatching of the larva, these
papillæ develop glands at their extremities, secreting a kind of
glutinous fluid[7]. After these papillæ have become formed cells first
make their appearance in the test; and there is simultaneously formed
a fresh inner cuticular layer of the test, to which at first the cells
are confined, though subsequently they are found in the outer layer
also. On the appearance of cells in the test the latter must be
regarded as a form, though a very abnormal one, of connective tissue.
When the tail of the larva has reached a very considerable length the
egg-membrane bursts, and the larva becomes free. The hatching takes
place in Asc. canina about 48-60 hours after impregnation. The free
larva (fig. 8 V.) has a swollen trunk, and a very long tail, which
soon becomes straightened out. It has a striking resemblance to a
tadpole (_vide_ fig. 10).

[7] It is probable that these papillæ are very primitive organs
of the Chordata. Structures, which are probably of the same
nature, are formed behind the mouth in the larvæ of Amphibia, and
in front of the mouth in the larvæ of Ganoids (Acipenser,
Lepidosteus), and are used by these larvæ for attaching
themselves.

In the free larval condition the Ascidians have in many respects a
higher organization than in the adult state. It is accordingly
convenient to divide the subsequent development into two periods, the
first embracing the stages from the condition represented in fig. 8 V.
up to the full development of the free larva, and the second the
period from the full development of the larva to the attainment of the
fixed adult condition.

_Growth and Structure of the free larva._

The nervous system. The nervous system was left as a closed tube
consisting of a dilated anterior division, and a narrow posterior one.
The former may be spoken of as the brain, and the latter as the spinal
cord; although the homologies of these two parts are quite uncertain.
The anterior part of the spinal cord lying within the trunk dilates
somewhat (fig. 8 V. and VI. _Rg_) and there may thus be distinguished
a trunk and a caudal section of the spinal cord.

[FIG. 9. LARVA OF ASCIDIA MENTULA. (From Gegenbaur; after Kupffer.)
Only the anterior part of the tail is represented.

_N´_. anterior swelling of neural tube; _N._ anterior swelling of
spinal portion of neural tube; _n._ hinder part of neural tube;
_ch._ notochord; _K._ branchial region of alimentary tract; _d._
oesophageal and gastric region of alimentary tract; _O._ eye; _a._
otolith; _o._ mouth; _s._ papilla for attachment.]

The original single vesicle of the brain becomes divided by the time
the larva is hatched into two sections (fig. 9)--(1) an anterior
vesicle with, for the most part, thin walls, in which unpaired
auditory and optic organs make their appearance, and (2) a posterior
nearly solid cephalic ganglion, through which there passes a narrow
continuation of the central canal of the nervous system. This ganglion
consists of a dorsal section formed of distinct cells, and a ventral
section formed of a punctated material with nuclei. The auditory
organ[8] consists of a 'crista acustica' (fig. 9), in the form of a
slight prominence of columnar cells on the ventral side of the
anterior cerebral vesicle; to the summit of which a spherical otolith
is attached by fine hairs. In the crista is a cavity containing clear
fluid. The dorsal half of the otolith is pigmented: the ventral half
is without pigment. The crista is developed _in situ_, but the otolith
is formed from a single cell on the dorsal side of the cerebral
vesicle, which forms a projection into the cavity of the vesicle, and
then travels (in a manner not clearly made out) round the right side
of the vesicle till it comes to the crista; to which it is at first
attached by a narrow pedicle. The fully developed eye (figs. 8 VI. and
9, _O_) consists of a cup-shaped retina, which forms a prominence
slightly on the right side of the posterior part of the dorsal wall of
the anterior cerebral vesicle, and of refractive media. The retina is
formed of columnar cells, the inner ends of which are imbedded in
pigment. The refractive media of the eye are directed towards the
cavity of the cerebral vesicle, and consist of a biconvex lens and a
meniscus. Half the lens is imbedded in the cavity of the retina and
surrounded by the pigment, and the other half is turned toward a
concavo-convex meniscus which corresponds in position with the cornea.
The development of the meniscus and lens is unknown, but the retina is
formed (fig. 8 V. _a_) as an outgrowth of the wall of the brain. At
the inner ends of the cells of this outgrowth a deposit of pigment
appears.

[8] For a fuller account of the organs of sense _vide_ the
chapters on the eye and ear.

The trunk section of the spinal cord (fig. 9, _N_) is separated by a
sharp constriction from the brain. It is formed of a superficial layer
of longitudinal nervous fibres, and a central core of ganglion cells.
The layer of fibres diminishes in thickness towards the tail, and
finally ceases to be visible. Kupffer detected three pairs of nerves
passing off from the spinal cord to the muscles of the tail. The
foremost of these arises at the boundary between the trunk and the
tail, and the two others at regular intervals behind this point.

The mesoblast and muscular system. It has already been stated that the
lateral walls of the archenteron in the tail give rise to muscular
cells. These cells lie about three abreast, and appear not to increase
in number; so that with the growth of the tail they grow enormously in
length, and eventually become imperfectly striated. The mesoblast
cells at the hinder end of the trunk, close to its junction with the
tail, do not become converted into muscle cells, but give rise to
blood corpuscles; and the axial remnant of the archenteron undergoes a
similar fate. According to Kowalevsky the heart is formed during
larval life as an elongated closed sack on the right side of the
endostyle.

The notochord. The notochord was left as a rod formed of a single row
of cells, or in As. canina and some other forms of two rows, extending
from just within the border of the trunk to the end of the tail.

According to Kowalevsky, Kupffer, Giard, etc. the notochord undergoes
a further development which finds its only complete parallel amongst
Chordata in the doubtful case of Amphioxus.

There appear between the cells peculiar, highly refractive discs (fig.
8 V. _Chs_). These become larger and larger, and finally, after
pushing the remnants of the cells with their nuclei to the sides,
coalesce together to form a continuous axis of hyaline substance. The
remnants of the cells with their nuclei form a sheath round the
hyaline axis (fig. 8 VI. _ch._). Whether the axis is to be regarded as
formed of an intercellular substance, or of a differentiation of parts
of the cells is still doubtful. Kupffer inclines to the latter view:
the analogy of the notochord of higher types appears to me to tell in
favour of the former one.

The alimentary tract. The anterior part of the primitive archenteron
alone retains a lumen, and from this part the whole of the permanent
alimentary tract (mesenteron) becomes developed. The anterior part of
it grows upwards, and before hatching an involution of the epiblast on
the dorsal side, just in front of the anterior extremity of the
nervous system, meets and opens into this upgrowth, and gives rise to
the permanent mouth (fig. 8 V. _o_).

Kowalevsky states that a pore is formed at the front end of the
nervous tube leading into the mouth (fig. 8 V. and VI. _f_) which
eventually gives rise to the ciliated sack, which lies in the adult at
the junction between the mouth and the branchial sack. Kupffer however
was unable to find this opening; but Kowalevsky's observations are
confirmed by those of Salensky on Salpa.

From the hinder end of the alimentary sack an outgrowth directed
dorsalwards makes its appearance (figs. 8 V. and 9, _d_), from which
the oesophagus, stomach and intestine become developed. It at first
ends blindly. The remainder of the primitive alimentary sack gives
rise to the branchial sack of the adult. Just after the larva has
become hatched, the outgrowth to form the stomach and oesophagus, etc.
bends ventralwards and to the right, and then turns again in a dorsal
and left direction till it comes close to the dorsal surface, somewhat
to the left of and close to the hinder end of the trunk. The first
ventral loop of this part gives rise to the oesophagus, which opens
into the stomach; from this again the dorsally directed intestine
passes off.

On the ventral wall of the branchial sack there is formed a narrow
fold with thickened walls, which forms the endostyle. It ends
anteriorly at the stomodæum and posteriorly at the point where the
solid remnant of the archenteron in the tail was primitively
continuous with the branchial sack. The whole of the alimentary wall
is formed of a single layer of hypoblast cells.

A most important organ connected with the alimentary system still
remains to be dealt with, viz. the atrial or peribranchial cavity. The
first rudiments of it appear at about the time of hatching, in the
form of a pair of dorsal epiblastic involutions (fig. 8 V. _kl_), at
the level of the junction between the brain and the spinal cord. These
involutions grow inwards, and meet corresponding outgrowths of the
branchial sack, with which they fuse. At the junction between them is
formed an elongated ciliated slit, leading from the branchial sack
into the atrial cavity of each side. The slits so formed are the first
pair of branchial clefts. Behind the first pair of branchial clefts a
second pair is formed during larval life by a second outgrowth of the
branchial sack meeting the epiblastic atrial involutions (fig. 8 VI.
_1ks_ and _2ks_). The intestine at first ends blindly close to the
left atrial involution, but the anus becomes eventually formed by an
opening being established between the left atrial involution and the
intestine.

During the above described processes the test remains quite intact,
and is not perforated at the oral or the atrial openings.

_The retrogressive metamorphosis of the larva._

The development of the adult from the larva is, as has already been
stated, in the main a retrogressive metamorphosis. The stages in this
metamorphosis are diagrammatically shewn in figs. 10 and 11. It
commences with the attachment of the larva (fig. 10 A) which takes
place by one of the three papillæ. Simultaneously with the attachment
the larval tail undergoes a complete atrophy (fig. 10 B), so that
nothing is left of it but a mass of fatty cells situated close to the
point of the previous insertion of the tail in the trunk.

FIG. 10. DIAGRAM SHEWING THE MODE OF ATTACHMENT AND SUBSEQUENT
RETROGRESSIVE METAMORPHOSIS OF A LARVAL ASCIDIAN. (From Lankester.)

The nervous system also undergoes a very rapid retrogressive
metamorphosis; and the only part of it which persists would seem to be
the dilated portion of the spinal cord in the trunk (Kupffer, No. 28).

The three papillæ, including that serving for attachment, early
disappear, and the larva becomes fixed by a growth of the test to
foreign objects.

An opening appears in the test some time after the larva is fixed,
leading into the mouth, which then becomes functional. The branchial
sack at the same time undergoes important changes. In the larva it is
provided with only two ciliated slits, which open into the, at this
stage, paired atrial cavity (fig. 10).

The openings of the atrial cavity at first are shut off from
communication with the exterior by the test, but not long after the
larva becomes fixed, two perforations are formed in the test, which
lead into the openings of the two atrial cavities. At the same time
the atrial cavities dilate so as gradually to embrace the whole
branchial sack to which their inner walls attach themselves. Shortly
after this the branchial clefts rapidly increase in number[9].

[9] The account of the multiplication of the branchial clefts is
taken from Krohn's paper on Phallusia mammillata (No. 24), but
there is every reason to think that it holds true in the main for
simple Ascidians.

The increase of the branchial clefts is somewhat complicated. Between
the two primitive clefts two new ones appear, and then a third appears
behind the last cleft. In the interval between each branchial cleft is
placed a vascular branchial vessel (fig. 8 VI. _bb_). Soon a great
number of clefts become added in a row on each side of the branchial
sack. These clefts are small ciliated openings placed transversely
with reference to the long axis of the branchial sack, but only
occupying a small part of the breadth of each side. The intervals
dorsal and ventral to them are soon filled by series of fresh rows of
slits, separated from each other by longitudinal bars. Each side of
the branchial sack becomes in this way perforated by a number of small
openings arranged in rows, and separated by transverse and
longitudinal bars. The whole structure forms the commencement of the
branchial basketwork of the adult; the arrangement of which differs
considerably in structure and origin from the simple system of
branchial clefts of normal vertebrate types. At the junction of the
transverse and longitudinal bars papillæ are formed projecting into
the lumen of the branchial sack.

[FIG. 11. DIAGRAM OF A VERY YOUNG ASCIDIAN. (From Lankester.)]

After the above changes are far advanced towards completion, the
openings of the two atrial sacks gradually approximate in the dorsal
line, and finally coalesce to form the single atrial opening of the
adult. The two atrial cavities at the same time coalesce dorsally to
form a single cavity, which is continuous round the branchial sack,
except along the ventral line where the endostyle is present. The
atrial cavity, from its mode of origin as a pair of epiblastic
involutions[10], is clearly a structure of the same nature as the
branchial or atrial cavity of Amphioxus; and has nothing whatever to
do with the true body cavity.

[10] In the asexually produced buds of Ascidians the atrial cavity
appears, with the exception of the external opening, to be formed
from the primitive branchial sack. In the buds of Pyrosoma however
it arises independently. These peculiarities in the buds cannot
weigh against the embryonic evidence that the atrial cavity arises
from involutions of the epiblast, and they may perhaps be
partially explained by the fact that in the formation of the
visceral clefts outgrowths of the branchial sack meet the atrial
involutions.

It has already been stated that the anus opens into the original left
atrial cavity; when the two cavities coalesce the anus opens into the
atrial cavity in the median dorsal line.

Two of the most obscure points in the development are the origin of
the mesoblast in the trunk, and of the body cavity. Of the former
subject we know next to nothing, though it seems that the cells
resulting from the atrophy of the tail are employed in the nutrition
of the mesoblastic structures of the trunk.

The body cavity in the adult is well developed in the region of the
intestine, where it forms a wide cavity lined by an epithelioid
mesoblastic layer. In the region of the branchial sack it is reduced
to the vascular channels in the walls of the sack.

Kowalevsky believes the body cavity to be the original segmentation
cavity, but this view can hardly be regarded as admissible in the
present state of our knowledge. In some other Ascidian types a few
more facts about the mesoblast will be alluded to.

With the above changes the retrogressive metamorphosis is completed;
and it only remains to notice the change in position undergone in the
attainment of the adult state. The region by which the larva is
attached grows into a long process (fig. 10 B), and at the same time
the part carrying the mouth is bent upwards so as to be removed nearly
as far as possible from the point of attachment. By this means the
condition in the adult (fig. 11) is gradually brought about; the
original dorsal surface with the oral and atrial openings becoming the
termination of the long axis of the body, and the nervous system being
placed between the two openings.

The genus Molgula presents a remarkable exception amongst the simple
Ascidians in that, in some if not all the species belonging to it,
development takes place (Lacaze Duthiers 29 and 30, Kupffer 28) quite
directly and without larval metamorphosis.

The ova are laid either singly or adhering together, and are very
opaque. The segmentation (Lacaze Duthiers) commences by the formation
of four equal spheres, after which a number of small clear spheres are
formed which envelope the large spheres. The latter give rise to a
closed enteric sack, and probably also to a mass of cells situated on
the ventral side, which appear to be mesoblastic. The epiblast is
constituted of a single layer of cells which completely envelopes the
enteric sack and the mesoblast.

While the ovum is still within the chorion five peculiar processes of
epiblast grow out; four of which usually lie in the same sectional
plane of the embryo. They are contractile and contain prolongations of
the body cavity. Their relative size is very variable.

The nervous system is formed on the dorsal side of the embryo before
the above projections make their appearance, but, though it seems
probable that it originates in the same manner as in the more normal
forms, its development has not been worked out. As soon as it is
formed it consists of a nervous ganglion similar to that usually found
in the adult. The history of the mass of mesoblast cells has been
inadequately followed, but it continuously disappears as the heart,
excretory organs, muscles, etc. become formed. So far as can be
determined from Kupffer's descriptions the body cavity is primitively
parenchymatous--an indication of an abbreviated development--and does
not arise as a definite split in the mesoblast.

The primitive enteric cavity becomes converted into the branchial
sack, and from its dorsal and posterior corner the oesophagus, stomach
and intestine grow out as in the normal forms. The mouth is formed by
the invagination of a disc-like thickening of the epidermis in front
of the nervous system on the dorsal side of the body; and the atrial
cavity arises behind the nervous system by a similar process at a
slightly later period. The gill clefts opening into the atrial cavity
are formed as in the type of simple Ascidians described by Krohn.

The embryo becomes hatched not long after the formation of the oral
and atrial openings, and the five epiblastic processes undergo
atrophy. They are not employed in the attachment of the adult.

The larva when hatched agrees in most important points with the adult;
and is without the characteristic provisional larval organs of
ordinary forms; neither organs of special sense nor a tail becoming
developed. It has been suggested by Kupffer that the ventrally
situated mesoblastic mass is the same structure as the mass of
elements which results in ordinary types from the degeneration of the
tail. If this suggestion is true it is difficult to believe that this
mass has any other than a nutritive function.

The larva of Ascidia ampulloides described by P. van Beneden is
regarded by Kupffer as intermediate between the Molgula larva and the
normal type, in that the larval tail and notochord and a pigment spot
are first developed, while after the atrophy of these organs peculiar
processes like those of Molgula make their appearance.

Sedentaria. The development of the fixed composite Ascidians is, so
far as we know, in the main similar to that of the simple Ascidians.
The larvæ of Botryllus sometimes attain, while still in the free
state, a higher stage of development with reference to the number of
gill slits, etc. than that reached by the simple Ascidians, and in
some instances (Botryllus auratus _Metschnikoff_) eight conical
processes are found springing in a ring-like fashion around the trunk.
The presence of these processes has led to somewhat remarkable views
about the morphology of the group; in that they were regarded by
Kölliker, Sars, etc. as separate individuals, and it was supposed that
the product of each ovum was not a single individual, but a whole
system of individuals with a common cloaca.

The researches of Metschnikoff (No. 32), Krohn (No. 25), and Giard
(No. 12), etc. demonstrate that this paradoxical view is untenable,
and that each ovum only gives rise to a single embryo, while the
stellate systems are subsequently formed by budding.

Natantia. Our knowledge of the development of Pyrosoma is mainly due
to Huxley (No. 16) and Kowalevsky (No. 22). In each individual of a
colony of Pyrosoma only a single egg comes to maturity at one time.
This egg is contained in a capsule formed of a structureless wall
lined by a flattened epithelioid layer. From this capsule a duct
passes to the atrial cavity, which, though called the oviduct,
functions as an afferent duct for the spermatozoa.

The segmentation is meroblastic, and the germinal disc adjoins the
opening of the oviduct. The segmentation is very similar to that which
occurs in Teleostei, and at its close the germinal disc has the form
of a cap of cells, without a trace of stratification or of a
segmentation cavity, resting upon the surface of the yolk, which forms
the main mass of the ovum.

After segmentation the blastoderm, as we may call the layer of cells
derived from the germinal disc, rapidly spreads over the surface of
the yolk, and becomes divided into two layers, the epiblast and the
hypoblast. At the same time it exhibits a distinction into a central
clearer and a peripheral more opaque region. At one end of the
blastoderm, which for convenience sake may be spoken of as the
posterior end, a disc of epiblast appears, which is the first rudiment
of the nervous system, and on each side of the middle of the
blastoderm there arises an epiblastic involution. The epiblastic
involutions give rise to the atrial cavity.

These involutions rapidly grow in length, and soon form longish tubes,
opening at the surface by pores situated not far from the posterior
end of the blastoderm.

The blastoderm at this stage, as seen on the surface of the yolk, is
shewn in fig. 12 A. It is somewhat broader than long. The nervous
system is shewn at _n_, and _at_ points to an atrial tube. A
transverse section, through about the middle of this blastoderm, is
represented in fig. 12 B. The epiblast is seen above. On each side is
the section of an atrial tube (_at_). Below is the hypoblast which is
separated from the yolk especially in the middle line; at each side it
is beginning to grow in below, on the surface of the yolk. The space
below the hypoblast is the alimentary cavity, the ventral wall of
which is formed by the cells growing in at the sides. Between the
epiblast and hypoblast are placed scattered mesoblast cells, the
origin of which has not been clearly made out.

[FIG. 12.

A. SURFACE VIEW OF THE OVUM OF PYROSOMA NOT FAR ADVANCED IN
DEVELOPMENT. The embryonic structures are developed from a disc-like
blastoderm.
B. TRANSVERSE SECTION THROUGH THE MIDDLE PART OF THE SAME
BLASTODERM.

_at._ atrial cavity; _hy._ hypoblast; _n._ nervous disc in the
region of the future Cyathozooid.]

In a later stage the openings of the two atrial tubes gradually travel
backwards, and at the same time approximate, till finally they meet
and coalesce at the posterior end of the blastoderm behind the nervous
disc (fig. 13, _cl_). The tubes themselves at the same time become
slightly constricted not far from their hinder extremities, and so
divided into a posterior region nearly coterminous with the nervous
system (fig. 13), and an anterior region. These two regions have very
different histories in the subsequent development.

The nervous disc has during these changes become marked by a median
furrow (fig. 13, _ng_), which is soon converted into a canal by the
same process as in the simple Ascidians. The closure of the groove
commences posteriorly and travels forwards. These processes are
clearly of the same nature as those which take place in Chordata
generally in the formation of the central nervous system.

In the region of the germinal disc which contains the anterior part of
the atrial tubes, the alimentary cavity becomes, by the growth of the
layer of cells described in the last stage, a complete canal, on the
outer wall of which the endostyle is formed as a median fold. The
whole anterior part of the blastoderm becomes at the same time
gradually constricted off from the yolk.

[FIG. 13. BLASTODERM OF PYROSOMA SHORTLY BEFORE ITS DIVISION INTO
CYATHOZOOID AND ASCIDIOZOOIDS. (After Kowalevsky.)

_cl._ cloacal (atrial) opening; _en._ endostyle; _at._ atrial
cavity; _ng._ nervous groove.

The heart and pericardial cavity are seen to the left.]

The fate of the anterior and posterior parts of the blastoderm is very
different. The anterior part becomes segmented into four zooids or
individuals, called by Huxley Ascidiozooids, which give rise to a
fresh colony of Pyrosoma. The posterior part forms a rudimentary
zooid, called by Huxley Cyathozooid, which eventually atrophies. These
five zooids are formed by a process of embryonic fission. This fission
commences by the appearance of four transverse constrictions in the
anterior part of the blastoderm; by which the whole blastoderm becomes
imperfectly divided into five regions, fig. 14 A.

The hindermost constriction (uppermost in my figure) lies just in
front of the pericardial cavity; and separates the Cyathozooid from
the four ascidiozooids. The three other constrictions mark off the
four Ascidiozooids. The Cyathozooid remains for its whole length
attached to the blastoderm, which has now nearly enveloped the yolk.
It contains the whole of the nervous system (_ng_), which is covered
behind by the opening of the atrial tubes (_cl_). The alimentary tract
in the Cyathozooid forms a tube with very delicate walls. The
pericardial cavity is completely contained within the Cyathozooid, and
the heart itself (_ht_) has become formed by an involution of the
walls of the cavity.

The Ascidiozooids are now completely separated from the yolk. They
have individually the same structure as the undivided rudiment from
which they originated; so that the organs they possess are simply two
atrial tubes, an alimentary tract with an endostyle, and
undifferentiated mesoblast cells.

In the following stages the Ascidiozooids grow with great rapidity.
They soon cease to lie in a straight line, and eventually form a ring
round the Cyathozooid and attached yolk sack.

While these changes are being accomplished in the external form of the
colony, both the Cyathozooids and the Ascidiozooids progress
considerably in development. In the Cyathozooid the atrial spaces
gradually atrophy, with the exception of the external opening, which
becomes larger and more conspicuous. The heart at the same time comes
into full activity and drives the blood through the whole colony. The
yolk becomes more and more enveloped by the Cyathozooid, and is
rapidly absorbed; while the nutriment derived from it is transported
to the Ascidiozooids by means of the vascular connection. The nervous
system retains its previous condition; and round the Cyathozooid is
formed the test into which cells migrate, and arrange themselves in
very conspicuous hexagonal areas. The delicate alimentary tract of the
Cyathozooid is still continuous with that of the first Ascidiozooid.
After the Cyathozooid has reached the development just described it
commences to atrophy.

[FIG. 14. TWO STAGES IN THE DEVELOPMENT OF PYROSOMA IN WHICH THE
CYATHOZOOID AND FOUR ASCIDIOZOOIDS ARE ALREADY DISTINCTLY FORMED.
(After Kowalevsky.)

_cy._ cyathozooid; _as._ ascidiozooid; _ng._ nervous groove; _ht._
heart of cyathozooid; _cl._ cloacal opening.]

The changes in the Ascidiozooids are even more considerable than those
in the Cyathozooid. A nervous system appears as a fresh formation
close to the end of each Ascidiozooid turned towards the Cyathozooid.
It forms a tube of which the open front end eventually develops into
the ciliated pit of the mouth, and the remainder into the actual
nervous ganglion. Between the nervous system and the endostyle an
involution appears, which gives rise to the mouth. On each side of the
primitive alimentary cavity of each Ascidiozooid branchial slits make
their appearance, leading into the atrial tubes; so that the primitive
alimentary tract becomes converted into the branchial sacks of the
Ascidiozooids. The remainder of the alimentary tract of each zooid is
formed as a bud from the hind end of the branchial sack in the usual
way. The alimentary tracts of the four Ascidiozooids are at first in
free communication by tubes opening from the hinder extremity of one
zooid into the dorsal side of the branchial sack of the next zooid. At
the hinder end of each Ascidiozooid is developed a mass of fatty cells
known as the elæoblast, which probably represents a rudiment of the
larval tail of simple Ascidians. (Cf. pp. 30-32.)

The further changes consist in the gradual atrophy of the Cyathozooid,
which becomes more and more enclosed within the four Ascidiozooids.
These latter become completely enveloped in a common test, and form a
ring round the remains of the yolk and of the Cyathozooid, the heart
of which continues however to beat vigorously. The cloacal opening of
the Cyathozooid persists through all these changes, and, after the
Cyathozooid itself has become completely enveloped in the
Ascidiozooids and finally absorbed, deepens to form the common cloacal
cavity of the Pyrosoma colony.

The main parts of the Ascidiozooids were already formed during the
last stage. The zooids long remain connected together, and united by a
vascular tube with the Cyathozooid, and these connections are not
severed till the latter completely atrophies. Finally, after the
absorption of the Cyathozooid, the Ascidiozooids form a rudimentary
colony of four individuals enveloped in a common test. The two atrial
tubes of each zooid remain separate in front but unite posteriorly. An
anus is formed leading from the rectum into the common posterior part
of the atrial cavity; and an opening is established between the
posterior end of the atrial cavity of each Ascidiozooid and the common
axial cloacal cavity of the whole colony. The atrial cavities in
Pyrosoma are clearly lined by epiblast, just as in simple Ascidians.

When the young colony is ready to become free, it escapes from the
atrial cavity of the parent, and increases in size by budding.

Doliolidæ. The sexually developed embryos of Doliolum have been
observed by Krohn (No. 23), Gegenbaur (No. 10), and Keferstein and
Ehlers (No. 17); but the details of the development have been very
imperfectly investigated.

The youngest embryo observed was enveloped in a large oval transparent
covering, the exact nature of which is not clear. It is perhaps a
larval rudiment of the test which would seem to be absent in the
adult. Within this covering is the larva, the main organs of which are
already developed; and which primarily differs from the adult in the
possession of a larval tail similar to that of simple Ascidians.

In the body both oral and atrial openings are present, the latter on
the dorsal surface; and the alimentary tract is fully established. The
endostyle is already formed on the ventral wall of the branchial sack,
but the branchial slits are not present. Nine muscular rings are
already visible. The tail, though not so developed as in the simple
Ascidians, contains an axial notochord of the usual structure, and
lateral muscles. It is inserted on the ventral side, and by its slow
movements the larva progresses.

In succeeding stages the tail gradually atrophies, and the gill slits,
four in number, develop; at the same time a process or stolon,
destined to give rise by budding to a second non-sexual generation,
makes its appearance on the dorsal side in the seventh intermuscular
space. This stolon is comparable with that which appears in the embryo
of Salpa. When the tail completely atrophies the larva leaves its
transparent covering, and becomes an asexual Doliolum with a dorsal
stolon.

Salpidæ. As is well known the chains of Salpa alone are sexual, and
from each individual of the chain only a single embryo is produced.
The ovum from which this embryo takes its origin is visible long
before the separate Salps of the chain have become completely
developed. It is enveloped in a capsule continuous with a duct, which
opens into the atrial cavity, and is usually spoken of as the oviduct.
The capsule with the ovum is enveloped in a maternal blood sinus.
Embryonic development commences after the chain has become broken up,
and the spermatozoa derived from another individual would seem to be
introduced to the ovum through the oviduct.

At the commencement of embryonic development the oviduct and
ovicapsule undergo peculiar changes; and in part at least give rise to
a structure subservient to the nutrition of the embryo, known as the
placenta. These changes commence with the shortening of the oviduct,
and the disappearance of a distinction between oviduct and ovicapsule.
The cells lining the innermost end of the capsule, _i.e._ that at the
side of the ovum turned away from the atrial cavity, become at the
same time very columnar. The part of the oviduct between the ovum and
the atrial cavity dilates into a sack, communicating on the one hand
with the atrial cavity, and on the other by a very narrow opening with
the chamber in which the egg is contained. This sack next becomes a
prominence in the atrial cavity, and eventually constitutes a
brood-pouch. The prominence it forms is covered by the lining of the
atrial cavity, immediately within which is the true wall of the sack.
The external opening of the sack becomes gradually narrowed, and
finally disappears. In the meantime the chamber in which the embryo is
at first placed acquires a larger and larger opening into the sack;
till finally the two chambers unite, and a single brood-pouch
containing the embryo is thus produced. The inner wall of the chamber
is formed by the columnar cells already spoken of. They form the
rudiment of the placenta. The double wall of the outer part of the
brood-pouch becomes stretched by the growth of the embryo; the inner
of its two layers then atrophies. The outer layer subsequently gives
way, and becomes rolled back so as to lie at the inner end of the
embryo, leaving the latter projecting freely into the atrial cavity.

While these changes are taking place the placenta becomes fully
developed. The first rudiment of it consists, according to Salensky,
of the thickened cells of the ovicapsule only, though this view is
dissented from by Brooks, Todaro, etc. Its cells soon divide to form a
largish mass, which becomes attached to a part of the epiblast of the
embryo.

On the formation of the body cavity of the embryo a central axial
portion of the placenta becomes separated from a peripheral layer; and
a channel is left between them which leads from a maternal blood sinus
into the embryonic body cavity. The peripheral layer of the placenta
is formed of cells continuous with the epiblast of the embryo; while
the axial portion is constituted of a disc of cells adjoining the
embryo, with a column of fibres attached to the maternal side. The
fibres of this column are believed by Salensky to be products of the
original rudiment of the placenta. The placenta now assumes a more
spherical form, and its cavity becomes shut off from the embryonic
body cavity. The fibrous column breaks up into a number of strands
perforating the lumen of the organ, and the cells of the wall become
stalked bodies projecting into the lumen.

When the larva is nearly ready to become free the placenta atrophies.

The placenta functions in the nutrition of the embryo in the following
way. It projects from its first formation into a maternal blood sinus,
and, on the appearance of a cavity in it continuous with the body
cavity of the embryo, the blood of the mother fully intermingles with
that of the embryo. At a later period the communication with the body
cavity of the embryo is shut off, but the cavity of the placenta is
supplied with a continuous stream of maternal blood, which is only
separated from the foetal blood by a thin partition.

It is now necessary to turn to the embryonic development about which
it is unfortunately not as yet possible to give a completely
satisfactory account. The statements of the different investigators
contradict each other on most fundamental points. I have followed in
the main Salensky (No. 34), but have also called attention to some
points where his observations diverge most from those of other
writers, or where they seem unsatisfactory.

The development commences at about the period when the brood-pouch is
becoming formed; and the ovum passes entirely into the brood-pouch
before the segmentation is completed. The segmentation is regular, and
the existence of a segmentation cavity is denied by Salensky, though
affirmed by Kowalevsky and Todaro[11].

[11] From Todaro's latest paper (No. 39) it would seem the
segmentation cavity has very peculiar relations.

At a certain stage in the segmentation the cells of the ovum become
divided into two layers, an epiblast investing the whole of the ovum
with the exception of a small area adjoining the placenta, where the
inner layer or hypoblast, which forms the main mass of the ovum,
projects at the surface. The epiblast soon covers the whole of the
hypoblast, so that there would seem (according to Salensky's
observations) to be a kind of epibolic invagination: a conclusion
supported by Todaro's figures.

At a later stage, on one side of the free apex of the embryo, a
mesoblastic layer makes its appearance between the epiblast and
hypoblast. This layer is derived by Salensky, as it appears to me on
insufficient grounds, from the epiblast. Nearly at the same time there
arises not far from the same point of the embryo, but on the opposite
side, a solid thickening of epiblast which forms the rudiment of the
nervous system. The nervous system is placed close to the front end of
the body; and nearly at the opposite pole, and therefore at the hind
end, there appears immediately below the epiblast a mass of cells
forming a provisional organ known as the elæoblast. Todaro regards
this organ as mesoblastic in origin, and Salensky as hypoblastic. The
organ is situated in the position which would be occupied by the
larval tail were it developed. It may probably be regarded (Salensky)
as a disappearing rudiment of the tail, and be compared in this
respect with the more or less similar mass of cells described by
Kupffer in Molgula, and with the elæoblast in Pyrosoma.

After the differentiation of these organs a cavity makes its
appearance between the epiblast and hypoblast, which is regarded by
Salensky as the body cavity. It appears to be equivalent to the
segmentation cavity of Todaro. According to Todaro's statements, it is
replaced by a second cavity, which appears between the splanchnic and
somatic layers of mesoblast, and constitutes the true body cavity. The
embryo now begins to elongate, and at the same time a cavity makes its
appearance in the centre of the hypoblast cells. This cavity is the
rudiment of the branchial and alimentary cavities: on its dorsal wall
is a median projection, the rudiment of the so-called gill of Salpa.

At two points this cavity comes into close contact with the external
skin. At one of these, situated immediately ventral to the nervous
system, the mouth becomes formed at a later period. At the other,
placed on the dorsal surface between the nervous system and the
elæoblast, is formed the cloacal aperture.

By the stage under consideration the more important systems of organs
are established, and the remaining embryonic history may be very
briefly narrated.

The embryo at this stage is no longer covered by the walls of the
brood-pouch but projects freely into the atrial cavity, and is only
attached to its parent by means of the placenta. The epiblast cells
soon give rise to a deposit which forms the mantle. The deposit
appears however to be formed not only on the outer side of the
epiblast but also on the inner side; so that the epiblast becomes
cemented to the subjacent parts, branchial sack, etc., by an
intercellular layer, which would seem to fill up the primitive body
cavity with the exception of the vascular channels (Salensky).

The nervous system, after its separation from the epiblast, acquires a
central cavity, and subsequently becomes divided into three lobes,
each with an internal protuberance. At its anterior extremity it opens
into the branchial sack; and from this part is developed the ciliated
pit of the adult. The nervous ganglion at a later period becomes
solid, and a median eye is subsequently formed as an outgrowth from
it.

According to Todaro there are further formed two small auditory (?
olfactory) sacks on the ventral surface of the brain, each of them
placed in communication with the branchial cavity by a narrow canal.

The mesoblast gives rise to the muscles of the branchial sack, to the
heart, and to the pericardium. The two latter are situated on the
ventral side of the posterior extremity of the branchial cavity.

_Branchial sack and alimentary tract._ The first development of the
enteric cavity has already been described. The true alimentary tract
is formed as a bud from the hinder end of the primitive cavity. The
remainder of the primitive cavity gives rise to the branchial sack.
The so-called gill has at first the form of a lamella attached
dorsally to the walls of the branchial sack; but its attachment
becomes severed except at the two ends, and it then forms a band
stretching obliquely across the branchial cavity, which subsequently
becomes hollow and filled with blood corpuscles. The whole structure
is probably homologous with the peculiar fold, usually prolonged into
numerous processes, which normally projects from the dorsal wall of
the Ascidian branchial sack.

On the completion of the gill the branchial sack becomes divided into
a region dorsal to the gill, and a region ventral to it. Into the
former the single atrial invagination opens. No gill slits are formed
comparable with those in simple Ascidians, and the only representative
of these structures is the simple communication which becomes
established between the dorsal division of the branchial sack and the
atrial opening. The whole branchial sack of Salpa, including both the
dorsal and ventral divisions, corresponds with the branchial sack of
simple Ascidians. On its ventral side the endostyle is formed in the
normal way. The mouth arises at the point already indicated near the
front end of the nervous system[12].

[12] Brooks takes a very different view of the nature of the
parts in Salpa. He says, No. 7, p. 322, "The atrium of Salpa,
when first observed, was composed of two broad lateral atria
within the body cavity, one on each side of the branchial sack,
and a very small mid-atrium.... The lateral atria do not however,
as in most Tunicata, remain connected with the mid-atrium, and
unite with the wall of the branchial sack to form the branchial
slits, but soon become entirely separated, and the two walls of
each unite so as to form a broad sheet of tissue, which soon
splits up to form the muscular bands of the branchial sack."
Again, p. 324, "During the changes which have been described as
taking place in the lateral atria, the mid-atrium has increased
in size.... The branchial and atrial tunics now unite upon each
side, so that the sinus is converted into a tube which
communicates, at its posterior end, with the heart and
perivisceral sinus, and at the anterior end with the neural
sinus. This tube is the gill.... The centres of the two regions
upon the sides of the gill, where these two tissues have become
united, are now absorbed, so that a single long and narrow
branchial slit is produced on each side of the gill. The
branchial cavity is thus thrown into communication with the
atrium, and the upper surface of the latter now unites with the
outer tunic, and the external atrial opening is formed by
absorption."

The above description would imply that the atrial cavity is a
space lined by mesoblast, a view which would upset the whole
morphology of the Ascidians. Salensky's account, which implies
only an immense reduction in the size of the atrial cavity as
compared with other types, appears to me far more probable. The
lateral atria of Brooks appear to be simply parts of the body
cavity, and have certainly no connection with the lateral atria
of simple Ascidians or Pyrosoma.

The observations of Todaro upon Salpa (No. 38) are very
remarkable, and illustrated by beautifully engraved plates. His
interpretations do not however appear quite satisfactory. The
following is a brief statement of some of his results.

During segmentation there arises a layer of small superficial
cells (epiblast) and a central layer of larger cells, which
becomes separated from the former by a segmentation cavity,
except at the pole adjoining the free end of the brood-pouch. At
this point the epiblast cells become invaginated into the central
cells and form the alimentary tract, while the primitive central
cells remain as the mesoblast. A fold arises from the epiblast
which Todaro compares to the vertebrate amnion, but the origin of
it is unfortunately not satisfactorily described. The folds of
the amnion project towards the placenta, and enclose a cavity
which, as the folds never completely meet, is permanently open to
the maternal blood sinus. This cavity corresponds with the cavity
of the true amnion of higher Vertebrates. It forms the cavity of
the placenta already described. Between the two folds of the
amnion is a cavity corresponding with the vertebrate false
amnion. A structure regarded by Todaro as the notochord is formed
on the neck, connecting the involution of the alimentary tract
with the exterior. It has only a very transitory existence.

In the later stages the segmentation cavity disappears and a true
body cavity is formed by a split in the mesoblast.

Todaro's interpretations, and in part his descriptions also, both
with reference to the notochord and amnion, appear to me quite
inadmissible. About some other parts of his descriptions it is
not possible to form a satisfactory judgment. He has recently
published a short paper on this subject (No. 39) preliminary to a
larger memoir, which is very difficult to understand in the
absence of plates. He finds however in the placenta various parts
which he regards as homologous with the decidua vera and reflexa
of Mammalia.

Development of the chain of sexual Salps. My description of the
embryonic development of Salpa would not be complete without some
reference to the development of the stolon of the Solitary generation
of Salps by the segmentation of which a chain of sexual Salps
originates.

The asexual Salp, the embryonic development of which has just been
described, may be compared to the Cyathozooid of Pyrosoma, from which
it mainly differs in being fully developed. While still in an
embryonic condition it gives rise to a process or stolon, which
becomes divided into a number of zooids by transverse constrictions,
in the same manner that part of the germ of the ovum of Pyrosoma is
divided by transverse constrictions into four Ascidiozooids.

The stolon arises as a projection on the right side of the body of the
embryo close to the heart. It is formed (Salensky, No. 35) of an
outgrowth of the body wall, into which there grow the following
structures:

(1) A central hollow process from the end of the respiratory sack.
(2) A right and left lateral prolongation of the pericardial cavity.
(3) A solid process of cells on the ventral side derived from the
same mass of the cells as the elæoblast.
(4) A ventral and a dorsal blood sinus.

Besides these parts there appears on the dorsal side a hollow tube,
the origin of which is unknown, which gives rise to the nervous
system.

The hollow process of the respiratory sack is purely provisional, and
disappears without giving rise to any permanent structure. The right
and left prolongations of the pericardial cavity become solid and
eventually give origin to the mesoblast. The ventral process of cells
is the most important structure in the stolon in that it gives rise
both to the alimentary and respiratory sacks, and to the generative
organs of the sexual Salps. The stolon containing the organs just
enumerated becomes divided by transverse constrictions into a number
of rings. These rings do not long remain complete, but become
interrupted dorsally and ventrally. The imperfect rings so formed soon
overlap, and each of them eventually gives rise to a sexual Salp.
Although the stolon arises while the asexual Salp is still in an
embryonic condition, it does not become fully developed till long
after the asexual Salp has attained maturity.

Appendicularia. Our only knowledge of the development of
Appendicularia is derived from Fol's memoir on the group (No. 8). He
simply states that it develops, as far as he was able to follow, like
other Ascidians; and that the extremely minute size of the egg
prevented him from pursuing the subject. He also states that the pair
of pores leading from the branchial cavity to the exterior is
developed from epiblastic involutions meeting outgrowths of the wall
of the branchial sack.

_Metagenesis._

One of the most remarkable phenomena in connection with the life
history of many Ascidians is the occurrence of an alternation of
sexual and gemmiparous generations. This alternation appears to have
originated from a complication of the process of reproduction by
budding, which is so common in this group. The mode in which this very
probably took place will be best understood by tracing a series of
transitional cases between simple budding and complete alternations of
generations.

In the simpler cases, which occur in some Composita Sedentaria, the
process of budding commences with an outgrowth of the body wall into
the common test, containing a prolongation of part of the alimentary
tract[13].

[13] It is not within the scope of this work to enter into
details with reference to the process of budding. The reader is
referred on this head more especially to the papers of Huxley
(No. 16) and Kowalevsky (No. 22) on Pyrosoma, of Salensky (No.
35) on Salpa, and Kowalevsky (No. 21) on Ascidians generally. It
is a question of very great interest how budding first arose, and
then became so prevalent in these degenerate types of Chordata.
It is possible to suppose that budding may have commenced by the
division of embryos at an early stage of development, and have
gradually been carried onwards by the help of natural selection
till late in life. There is perhaps little in the form of budding
of the Ascidians to support this view--the early budding of
Didemnum as described by Gegenbaur being the strongest evidence
for it--but it fits in very well with the division of the embryo
in Lumbricus trapezoides described by Kleinenberg, and with the
not unfrequent occurrence of double monsters in Vertebrata which
may be regarded as a phenomenon of a similar nature (Rauber). The
embryonic budding of Pyrosoma, which might perhaps be viewed as
supporting the hypothesis, appears to me not really in favour of
it; since the Cyathozooid of Pyrosoma is without doubt an
extremely modified form of zooid, which has obviously been
specially developed in connection with the peculiar reproduction
of the Pyrosomidæ.

Between the epiblastic and hypoblastic layers of the bud so formed, a
mesoblastic and sometimes a generative outgrowth of the parent also
appears.

The systems of organs of the bud are developed from the corresponding
layers to those in the embryo[14]. The bud eventually becomes
detached, and in its turn gives rise to fresh buds. Both the bud and
its parent reproduce sexually as well as by budding: the new colonies
being derived from sexually produced embryos.

[14] The atrial spaces form somewhat doubtful exceptions to the
rule.

The next stage of complication is that found in Botryllus (Krohn, Nos.
25 and 26). The larva produced sexually gives rise to a bud from the
right side of the body close to the heart. On the bud becoming
detached the parent dies away without developing sexual organs. The
bud of the second generation gives rise to two buds, a right one and a
left one, and like the larva dies without reaching sexual maturity.
The buds of the third generation each produce two buds and then suffer
the same fate as their parent.

The buds of the third generation arrange themselves with their cloacal
extremities in contact, and in the fourth generation a common cloaca
is formed, and so a true radial system of zooids is established; the
zooids of which are not however sexual.

The buds of the fourth generation in their turn produce two or three
buds and then die away.

Fresh systems become formed by a continuation of the process of
budding, but the zooids of the secondary systems so formed are sexual.
The ova come to maturity before the spermatozoa, so that cross
fertilization takes place.

In Botryllus we have clearly a rudimentary form of alternations of
generations, in that the sexually produced larva is asexual, and,
after a series of asexual generations, produced gemmiparously, there
appear sexual generations, which however continue to reproduce
themselves by budding.

The type of alternations of generations observable in Botryllus
becomes, as pointed out by Huxley, still more marked in Pyrosoma.

The true product of the ovum is here (_vide_ p. 25) a rudimentary
individual called by Huxley the Cyathozooid. This gives rise, while
still an embryo, by a process equivalent to budding to four fully
developed zooids (Ascidiozooids) similar to the parent form, and
itself dies away. The four Ascidiozooids form a fresh colony, and
reproduce (1) sexually, whereby fresh colonies are formed, and (2) by
ordinary budding, whereby the size of the colony is increased. All the
individuals of the colony are sexual.

The alternation of generations in Pyrosoma widely differs from that in
Botryllus in the fact of the Cyathozooid differing so markedly in its
anatomical characters from the ordinary zooids.

In Salpa the process is slightly different[15]. The sexual forms are
_now incapable of budding_, and, although at first a series of sexual
individuals are united together in the form of a chain, so as to form
a colony like Pyrosoma or Botryllus, yet they are so loosely connected
that they separate in the adult state. As in Botryllus, the ova are
ripe before the spermatozoa. Each sexual individual gives rise to a
single offspring, which, while still in the embryonic condition, buds
out a 'stolon' from its right ventral side. This stolon is divided
into a series of lateral buds after the solitary asexual Salp has
begun to lead an independent existence. The solitary asexual Salp
clearly corresponds with the Cyathozooid of Pyrosoma, though it has
not, like the Cyathozooid, undergone a retrogressive metamorphosis.

[15] _Vide_ p. 33.

By far the most complicated form of alternation of generations known
amongst the Ascidians is that in Doliolum. The discovery of this
metamorphosis was made by Gegenbaur (No. 10). The sexual form of
Doliolum is somewhat cask-shaped, with ring-like muscular bands, and
the oral and atrial apertures placed at opposite ends of the cask. The
number of gill slits varies according to the species. The ovum gives
rise, as already described, to a tailed embryo which subsequently
develops into a cask-shaped asexual form. On attaining its full size
it loses its branchial sack and alimentary tract. While still in the
embryonic condition, a stolon grows out from its dorsal side in the
seventh intermuscular space. The stolon, like that in Salpa, contains
a prolongation of the branchial sack[16].

[16] I draw this conclusion from Gegenbaur's fig. (No. 10), Pl.
XVI., fig. 15. The body (_x_) in the figure appears to me without
doubt the rudiment of the stolon, and not, as believed by
Gegenbaur, the larval tail.

On this stolon there develop two entirely different types of buds, (1)
lateral buds, (2) dorsal median buds.

The lateral buds are developed in regular order on the two sides of
the stolon, and the most advanced buds are those furthest removed from
the base. They give rise to forms with a very different organization
to that of the parent. They are compared by Gegenbaur to a spoon, the
bowl of which is formed by the branchial sack, and the handle by the
stalk attaching the bud to the stolon. The oral opening into the
branchial sack is directed upwards: _an atrial opening is remarkably
enough not present_. The branchial sack is perforated by numerous
openings. It leads into an alimentary tract which opens directly to
the exterior by an anus opposite the mouth.

The stalks attaching the more mature buds to the stolon are provided
with ventrally directed scales, which completely hide the stolon in a
view from the ventral surface.

These buds have, even after their detachment, no trace of generative
organs, and shew no signs of reproducing themselves by budding. Their
eventual fate is unknown.

The median dorsal buds have no such regular arrangement as the lateral
buds, but arise in irregular bunches, those furthest removed from the
base of the stolon being however the oldest. These buds are almost
exactly similar to the original sexual form; they do not acquire
sexual organs, but are provided with a stolon attached on the ventral
side, in the sixth intermuscular space.

This stolon is simply the stalk by which each median bud was
primitively attached to the stolon of the first asexual form.

From the stolon of the median buds of the second generation buds are
developed which grow into the sexual forms.

The generations of Doliolum may be tabulated in the following way.

Sexual generation,
|
1st asexual form with dorsal stolon,
_________________|_____________________
| |
spoon-like forms developed as 2nd asexual forms developed as
lateral buds (eventual history median buds with ventral stolon,
unknown). |
sexual generation.

BIBLIOGRAPHY.

(6) P. J. van Beneden. "Recherches s. l'Embryogénie, l'Anat. et la
Physiol. des Ascidies simples." _Mém. Acad. Roy. de Belgique_, Tom.
XX.

(7) W. K. Brooks. "On the development of Salpa." _Bull. of the Museum
of Comp. Anat. at Harvard College, Cambridge, Mass._

(8) H. Fol. _Etudes sur les Appendiculaires du détroit de Messine_.
Genève et Bâle, 1872.

(9) Ganin. "Neue Thatsachen a. d. Entwicklungsgeschichte d. Ascidien."
_Zeit. f. wiss. Zool._, Vol. XX. 1870.

(10) C. Gegenbaur. "Ueber den Entwicklungscyclus von Doliolum nebst
Bemerkungen über die Larven dieser Thiere." _Zeit. f. wiss. Zool._,
Bd. VII. 1856.

(11) A. Giard. "Etudes critiques des travaux d'embryogénie relatifs à
la parenté des Vertebrés et des Tuniciers." _Archiv Zool.
expériment._, Vol. I. 1872.

(12) A. Giard. "Recherches sur les Synascidies." _Archiv Zool.
expér._, Vol. I. 1872.

(13) O. Hertwig. "Untersuchungen üb. d. Bau u. d. Entwicklung des
Cellulose-Mantels d. Tunicaten." _Jenaische Zeitschrift_, Bd. VII.
1873.

(14) Th. H. Huxley. "Remarks upon Appendicularia and Doliolum." _Phil.
Trans._, 1851.

(15) Th. H. Huxley. "Observations on the anatomy and physiology of
Salpa and Pyrosoma." _Phil. Trans._, 1851.

(16) Th. H. Huxley. "Anatomy and development of Pyrosoma." _Linnean
Trans._, 1860, Vol. XXIII.

(17) Keferstein u. Ehlers. _Zoologische Beiträge_, 1861. Doliolum.

(18) A. Kowalevsky. "Entwicklungsgeschichte d. einfachen Ascidien."
_Mém. Acad. Pétersbourg_, VII. série, T. X. 1866.

(19) A. Kowalevsky. "Beitrag z. Entwick. d. Tunicaten." _Nachrichten
d. königl. Gesell. zu Göttingen._ 1868.

(20) A. Kowalevsky. "Weitere Studien üb. d. Entwicklung d. einfachen
Ascidien." _Archiv f. mikr. Anat._, Vol. VII. 1871.

(21) A. Kowalevsky. "Ueber Knospung d. Ascidien." _Archiv f. mikr.
Anat._, Vol. X. 1874.

(22) A. Kowalevsky. "Ueber die Entwicklungsgeschichte d. Pyrosoma."
_Archiv f. mikr. Anat._, Vol. XI. 1875.

(23) A. Krohn. "Ueber die Gattung Doliolum u. ihre Arten." _Archiv f.
Naturgeschichte_, Bd. XVIII. 1852.

(24) A. Krohn. "Ueber die Entwicklung d. Ascidien." _Müller's Archiv_,
1852.

(25) A. Krohn. "Ueber die Fortpflanzungsverhältnisse d. Botrylliden."
_Archiv f. Naturgeschichte_, Vol. XXXV. 1869.

(26) A. Krohn. "Ueber die früheste Bildung d. Botryllenstöcke."
_Archiv f. Naturgeschichte_, Vol. XXXV. 1869.

(27) C. Kupffer. "Die Stammverwandschaft zwischen Ascidien u.
Wirbelthieren." _Archiv f. mikr. Anat._, Vol. VI. 1870.

(28) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." _Archiv f.
mikr. Anat._, Vol. VIII. 1872.

(29) H. Lacaze Duthiers. "Recherches sur l'organisation et
l'Embryogénie des Ascidies (Molgula tubulosa)." _Comptes rendus_, May
30, 1870, p. 1154.

(30) H. Lacaze Duthiers. "Les Ascidies simples des Côtes de France"
(Development of Molgula). _Archiv Zool. expér._, Vol. III. 1874.

(31) R. Leuckart. "Salpa u. Verwandte." _Zoologische Untersuchungen_,
Heft II.

(32) E. Metschnikoff. "Observations sur le développement de quelques
animaux (Botryllus and Simple Ascidians)." _Bull. d. l'Acad.
Pétersbourg_, Vol. XIII. 1869.

(33) H. Milne-Edwards. "Observations s. l. Ascidies composées des
côtes de la Manche." _Mémoires d. l'Institut_, T. XVIII. 1842.

(34) W. Salensky. "Ueber d. embryonale Entwicklungsgeschichte der
Salpen." _Zeit. f. wiss. Zool._, B. XXVII. 1877.

(35) W. Salensky. "Ueber die Knospung d. Salpen." _Morphol. Jahrbuch_,
Bd. III. 1877.

(36) W. Salensky. "Ueber die Entwicklung d. Hoden u. über den
Generationswechsel d. Salpen." _Zeit. f. wiss. Zool._, Bd. XXX. Suppl.
1878.

(37) C. Semper. "Ueber die Entstehung d. geschichteten
Cellulose-Epidermis d. Ascidien." _Arbeit. a. d. zool.-zoot. Instit.
Würzburg_, Vol. II. 1875.

(38) Fr. Todaro. _Sopra lo sviluppo e l'anatomia delle Salpe._ Roma,
1875.

(39) Fr. Todaro. "Sui primi fenomeni dello sviluppo delle Salpe."
_Reale Accademia dei Lincei_, Vol. IV. 1880.

CHAPTER III.

ELASMOBRANCHII.

The impregnation of the ovum is effected in the oviduct. In most forms
the whole of the subsequent development, till the time when the embryo
is capable of leading a free existence, takes place in the uterus; but
in other cases the egg becomes enveloped, during its passage down the
oviduct, first in a layer of fluid albumen, and finally in a dense
horny layer, which usually takes the form of a quadrilateral capsule
with characters varying according to the species. After the formation
of this capsule the egg is laid, and the whole of the development,
with the exception of the very first stages, takes place externally.

In many of the viviparous forms (Mustelus, Galeus, Carcharias,
Sphyrna) the egg is enclosed, during the early stages of development
at any rate, in a very delicate shell homologous with that of the
oviparous forms; there is usually also a scanty albuminous layer. Both
of these are stated by Gerbe (No. 42) to be absent in Squalus spinax.

The following are examples of viviparous genera: Hexanchus, Notidanus,
Acanthias, Scymnus, Galeus, Squalus, Mustelus, Carcharias, Sphyrna,
Squatina, Torpedo; and the following of oviparous genera: Scyllium,
Pristiurus, Cestracion, Raja[17].

[17] For further details, _vide_ Müller (No. 48).

The ovum at the time of impregnation has the form of a large spherical
mass, similar to the yolk of a bird's egg, but without a vitelline
membrane[18]. The greater part of it is formed of peculiar oval
spherules of food-yolk, held together by a protoplasmic network. The
protoplasm is especially concentrated in a small lens-shaped area,
known as the germinal disc, which is not separated by a sharp line
from the remainder of the ovum. Yolk spherules are present in this
disc as elsewhere, but are much smaller and of a different character.
The segmentation has the normal meroblastic character (fig. 15) and is
confined to the germinal disc. Before it commences the germinal disc
exhibits amoeboid movements. During the segmentation nuclei make their
appearance spontaneously (?) in the yolk adjoining the germinal disc
(fig. 15, _nx´_), and around them portions of the yolk with its
protoplasmic network become segmented off. Cells are thus formed which
are added to those resulting from the segmentation proper. Even after
the segmentation numerous nuclei are present in the granular matter
below the blastoderm (fig. 16 A, _n´_); and around these cells are
being continually formed, which enter the blastoderm, and are more
especially destined to give rise to the hypoblast. The special
destination of many of these cells is spoken of in detail below.

[18] _Vide_ Vol. II., p. 62.

[FIG. 15. SECTION THROUGH GERMINAL DISC OF A PRISTIURUS EMBRYO
DURING THE SEGMENTATION.

_n._ nucleus; _nx._ nucleus modified prior to division; _nx´._
modified nucleus in the yolk; _f._ furrow appearing in the yolk
adjacent to the germinal disc.]

At the close of segmentation the blastoderm forms a somewhat
lens-shaped disc, thicker at one end than at the other; the thicker
end being the embryonic end. It is divided into two strata--an upper
one, the epiblast--formed of a single row of columnar cells; and a
lower one, the primitive hypoblast, consisting of the remaining cells
of the blastoderm, and forming a mass several strata deep. These cells
will be spoken of as the lower layer cells, to distinguish them from
the true hypoblast which is one of their products.

[FIG. 16. TWO LONGITUDINAL SECTIONS OF THE BLASTODERM OF A
PRISTIURUS EMBRYO DURING STAGES PRIOR TO THE FORMATION OF THE
MEDULLARY GROOVE.

_ep._ epiblast; _ll._ lower layer cells or primitive hypoblast; _m._
mesoblast; _hy._ hypoblast; _sc._ segmentation cavity; _es._ embryo
swelling; _n´._ nuclei of yolk; _er._ embryonic rim. _c._ lower
layer cells at the non-embryonic end of the blastoderm.]

A cavity very soon appears in the lower layer cells, near the
non-embryonic end of the blastoderm, but the cells afterwards
disappear from the floor of this cavity, which then lies between the
yolk and the lower layer cells (fig. 16 A, _sc_). This cavity is the
segmentation cavity equivalent to that present in Amphioxus, Amphibia,
etc. The chief peculiarity about it is the relatively late period at
which it makes its appearance, and the fact that its roof is formed
both by the epiblast and by the lower layer cells. Owing to the large
size of the segmentation cavity the blastoderm forms a thin layer
above the cavity and a thickened ridge round its edge.

[FIG. 17. LONGITUDINAL SECTION THROUGH THE BLASTODERM OF A
PRISTIURUS EMBRYO OF THE SAME AGE AS FIG. 28 B.

_ep._ epiblast; _er._ embryonic rim; _m._ mesoblast; _al._
mesenteron.]

The epiblast in the next stage is inflected for a small arc at the
embryonic end of the blastoderm, where it becomes continuous with the
lower layer cells; at the same time some of the lower layer cells of
the embryonic end of the blastoderm assume a columnar form, and
constitute the true hypoblast. The portion of the blastoderm, where
epiblast and hypoblast are continuous, forms a projecting structure
which will be called the embryonic rim (fig. 16 B, _er_).

This rim is a very important structure, since it represents the dorsal
portion of the lip of the blastopore of Amphioxus. The space between
it and the yolk represents the commencing mesenteron, of which the
hypoblast on the under side of the lip is the dorsal wall. The ventral
wall of the mesenteron is at first formed solely of yolk held together
by a protoplasmic network with numerous nuclei. The cavity under the
lip becomes rapidly larger (fig. 17, _al_), owing to the continuous
conversion of lower layer cells into columnar hypoblast along an axial
line passing from the middle of the embryonic rim towards the centre
of the blastoderm. The continuous differentiation of the hypoblast
towards the centre of the blastoderm corresponds with the invagination
in Amphioxus. During the formation of the embryonic rim the blastoderm
grows considerably larger, but, with the exception of the formation of
the embryonic rim, retains its primitive constitution.

The segmentation cavity undergoes however important changes. There is
formed below it a floor of lower layer cells, derived partly from an
ingrowth from the two sides, but mainly from the formation of cells
around the nuclei of the yolk (fig. 16). Shortly after the floor of
cells has appeared, the whole segmentation cavity becomes obliterated
(fig. 17).

The disappearance of the segmentation cavity corresponds in point of
time with the formation of the hypoblast by the pseudo-invagination
above described; and is probably due to this pseudo-invagination, in
the same way that the disappearance of the segmentation cavity in
Amphioxus is due to the true invagination of the hypoblast.

When the embryonic rim first appears there are no external indications
of the embryo as distinguished from the blastoderm, but when it has
attained to some importance the position of the embryo becomes marked
out by the appearance of a shield-like area extending inwards from the
edge of the embryonic rim, and formed of two folds with a groove
between them (fig. 28 B, _mg_), which is deepest at the edge of the
blastoderm, and shallows out as it extends inwards. This groove is the
medullary groove; and its termination at the edge of the blastoderm is
placed at the hind end of the embryo.

At about the time of its appearance the mesoblast becomes first
definitely established.

[FIG. 18. TWO TRANSVERSE SECTIONS OF AN EMBRYO OF THE SAME AGE AS
FIG. 17.

A. Anterior section.
B. Posterior section.

_mg._ medullary groove; _ep._ epiblast; _hy._ hypoblast; _n.al._
cells formed round the nuclei of the yolk which have entered the
hypoblast; _m._ mesoblast.

The sections shew the origin of the mesoblast.]

At the edge of the embryonic rim the epiblast and lower layer cells
are continuous. Immediately underneath the medullary groove, as is
best seen in transverse section (fig. 18), the whole of the lower
layer cells become converted into hypoblast, and along this line the
columnar hypoblast is in contact with the epiblast above. At the sides
however this is not the case; but at the junction of the epiblast and
lower layer cells the latter remain undifferentiated. A short way from
the edge the lower layer cells become divided into two distinct
layers, a lower one continuous with the hypoblast in the middle line,
and an upper one between this and the epiblast (fig. 18 B). The upper
layer is the commencement of the mesoblast (_m_). The mesoblast thus
arises as two independent lateral plates, one on each side of the
medullary groove, which are continuous behind with the undifferentiated
lower layer cells at the edge of the embryonic rim. The mesoblast
plates are at first very short, and do not extend to the front end of
the embryo. They soon however grow forwards as two lateral ridges,
attached to the hypoblast, one on each side of the medullary groove
(fig. 18 A, _m_). These ridges become separate from the hypoblast, and
form two plates, thinner in front than behind; but still continuous at
the edge of the blastoderm with the undifferentiated cells of the lip
of the blastopore, and laterally with the lower layer cells of the
non-embryonic part of the blastoderm. It results from the above mode
of development of the mesoblast, that it may be described as arising
in the form of a pair of solid outgrowths of the wall of the
alimentary tract; which differ from the mesoblastic outgrowths of the
wall of the archenteron in Amphioxus in not containing a prolongation
of the alimentary cavity.

[FIG. 19. DIAGRAMMATIC LONGITUDINAL SECTIONS OF AN ELASMOBRANCH
EMBRYO.

_Epiblast_ without shading. _Mesoblast_ black with clear outlines to
the cells. _Lower layer cells_ and _hypoblast_ with simple shading.

_ep._ epiblast; _m._ mesoblast; _al._ alimentary cavity; _sg._
segmentation cavity; _nc._ neural canal; _ch._ notochord; _x._ point
where epiblast and hypoblast become continuous at the posterior end
of the embryo; _n._ nuclei of yolk.

A. Section of young blastoderm, with segmentation cavity enclosed in
the lower layer cells.

B. Older blastoderm with embryo in which hypoblast and mesoblast are
distinctly formed, and in which the alimentary slit has appeared.
The segmentation cavity is still represented as being present,
though by this stage it has in reality disappeared.

C. Older blastoderm with embryo in which the neural canal has become
formed, and is continuous posteriorly with the alimentary canal. The
notochord, though shaded like mesoblast, belongs properly to the
hypoblast.]

A general idea of the structure of the blastoderm at this stage may be
gathered from the diagram representing a longitudinal section through
the embryo (fig. 19 B). In this figure the epiblast is represented in
white and is seen to be continuous at the lip of the blastopore (_x_)
with the shaded hypoblast. Between the epiblast and hypoblast is seen
one of the lateral plates of mesoblast, represented by black cells
with clear outlines. The non-embryonic lower layer cells of the
blastoderm are represented in the same manner as the mesoblast of the
body. The alimentary cavity is shewn at _al_, and below it is seen the
yolk with nuclei (_n_). The segmentation cavity is represented as
still persisting, though by this stage it would have disappeared.

[FIG. 20. THREE SECTIONS THROUGH A PRISTIURUS EMBRYO SOMEWHAT
YOUNGER THAN FIG. 28 C.

A. Section through the cephalic plate.
B. Section through the posterior part of the cephalic plate.
C. Section through the trunk.

_ch._ notochord; _mg._ medullary groove; _al._ alimentary tract;
_lp._ lateral plate of mesoblast; _pp._ body cavity.]

As to the growth of the blastoderm it may be noted that it has greatly
extended itself over the yolk. Its edge in the meantime forms a marked
ridge, which is due not so much to a thickening as to an arching of
the epiblast. This ridge is continuous with the embryonic rim, which
gradually concentrates itself into two prominences, one on each side
of the tail of the embryo, mainly formed of masses of undifferentiated
lower layer cells. These prominences will be called the caudal
swellings.

By this stage the three layers of the body, the epiblast, mesoblast,
and hypoblast, have become definitely established. The further history
of these layers may now be briefly traced.

Epiblast. While the greater part of the epiblast becomes converted
into the external epidermis, from which involutions give rise to the
olfactory and auditory pits, the lens of the eye, the mouth cavity,
and anus, the part of it lining the medullary groove becomes converted
into the central nervous system and optic cup. The medullary groove is
at first continued to the front end of the medullary plate; but the
anterior part of this plate soon enlarges, and the whole plate assumes
a spatula form (fig. 28 C, _h_, and fig. 20 A and B). The enlarged
part becomes converted into the brain, and may be called the cephalic
plate.

The posterior part of the canal deepens much more rapidly than the
rest (fig. 20 C), and the medullary folds unite dorsally and convert
the posterior end of the medullary groove into a closed canal, while
the groove is still widely open elsewhere. The medullary canal does
not end blindly behind, but simply forms a tube not closed at either
extremity. The importance of this fact will appear later.

Shortly after the medullary folds have met behind the whole canal
becomes closed in. This occurs in the usual way by the junction and
coalescence of the medullary folds. In the course of the closing of
the medullary groove the edges of the cephalic plate, which have at
first a ventral curvature, become bent up in the normal manner, and
enclose the dilated cephalic portion of the medullary canal. The
closing of the medullary canal takes place earlier in the head and
neck than in the back.

An anterior pore at the front end of the canal, like that in Amphioxus
and the Ascidians, is not found. The further differentiation of the
central nervous system is described in a special chapter: it may
however here be stated that the walls of the medullary canal give rise
not only to the central nervous system but to the peripheral also.

Mesoblast. The mesoblast was left as two lateral plates continuous
behind with the undifferentiated cells of the caudal swellings.

The cells composing them become arranged in two layers (fig. 20 C,
_lp_), a splanchnic layer adjoining the hypoblast, and a somatic layer
adjoining the epiblast. Between these two layers there is soon
developed in the region of the head a well-marked cavity (fig. 20 A,
_pp_) which is subsequently continued into the region of the trunk,
and forms the primitive body cavity, equivalent to the cavity
originating as an outgrowth of the archenteron in Amphioxus. The body
cavities of the two sides are at first quite independent.

[FIG. 21. TRANSVERSE SECTION THROUGH THE TRUNK OF AN EMBRYO SLIGHTLY
OLDER THAN FIG. 28 E.

_nc._ neural canal; _pr._ posterior root of spinal nerve; _x._
subnotochordal rod; _ao._ aorta; _sc._ somatic mesoblast; _sp._
splanchnic mesoblast; _mp._ muscle-plate; _mp´._ portion of
muscle-plate converted into muscle; _Vv._ portion of the vertebral
plate which will give rise to the vertebral bodies; _al._ alimentary
tract.]

Coincidentally with the appearance of differentiation into somatic and
splanchnic layers the mesoblast plates become in the region of the
trunk partially split by a series of transverse lines of division into
mesoblastic somites. Only the dorsal parts of the plates become split
in this way, their ventral parts remaining quite intact. As a result
of this each plate becomes divided into a dorsal portion adjoining the
medullary canal, which is divided into somites, and may be called the
_vertebral plate_, and a ventral portion not so divided, which may be
called the _lateral plate_. These two parts are at this stage quite
continuous with each other; and the body cavity originally extends
uninterruptedly to the summit of the vertebral plates (fig. 21).

[FIG. 22. HORIZONTAL SECTION THROUGH THE TRUNK OF AN EMBRYO OF
SCYLLIUM CONSIDERABLY YOUNGER THAN 28 F.

The section is taken at the level of the notochord, and shews the
separation of the cells to form the vertebral bodies from the
muscle-plates.

_ch._ notochord; _ep._ epiblast; _Vr._ rudiment of vertebral body;
_mp._ muscle-plate; _mp´._ portion of muscle-plate already
differentiated into longitudinal muscles.]

The next change results in the complete separation of the vertebral
portion of the plate from the lateral portion; thereby the upper
segmented part of the body cavity becomes isolated, and separated from
the lower and unsegmented part. As a consequence of this change the
vertebral plate comes to consist of a series of rectangular bodies,
the mesoblastic somites, each composed of two layers, a somatic and a
splanchnic, between which is the cavity originally continuous with the
body cavity (fig. 23, _mp_). The splanchnic layer of the plates buds
off cells to form the rudiments of the vertebral bodies which are at
first segmented in the same planes as the mesoblastic somites (fig.
22, _Vr_). The plates themselves remain as the muscle-plates (_mp_),
and give rise to the whole of the voluntary muscular system of the
body. Between the vertebral and lateral plates there is left a
connecting isthmus, with a narrow prolongation of the body cavity
(fig. 23 B, _st_), which gives rise (as described in a special
chapter) to the segmental tubes and to other parts of the excretory
system.

In the meantime the lateral plates of the two sides unite ventrally
throughout the intestinal and cardiac regions of the body, and the two
primitively isolated cavities contained in them coalesce. In the tail
however the plates do not unite ventrally till somewhat later, and
their contained cavities remain distinct till eventually obliterated.

At first the pericardial cavity is quite continuous with the body
cavity; but it eventually becomes separated from the body cavity by
the attachment of the liver to the abdominal wall, and by a horizontal
septum in which run the two ductus Cuvieri (fig. 23 A, _sv_). Two
perforations in this septum (fig. 23 A) leave the cavities in
permanent communication.

The parts derived from the two layers of the mesoblast (not including
special organs or the vascular system) are as follows:--

From the somatic layer are formed

(1) A considerable part of the voluntary muscular system of the
body.
(2) The dermis.
(3) A large part of the intermuscular connective tissue.
(4) Part of the peritoneal epithelium.

From the splanchnic layer are formed

(1) A great part of the voluntary muscular system.
(2) Part of the intermuscular connective tissue.
(3) The axial skeleton and surrounding connective tissue.
(4) The muscular and connective-tissue wall of the alimentary tract.
(5) Part of the peritoneal epithelium.

[FIG. 23. SECTIONS THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY
YOUNGER THAN 28 F.

Figure A shews the separation of the body cavity from the
pericardial cavity by a horizontal septum in which runs the ductus
Cuvieri; on the left side is seen the narrow passage which remains
connecting the two cavities. Fig. B through a posterior part of the
trunk shews the origin of the segmental tubes and of the primitive
ova.

_sp.c._ spinal canal; _W._ white matter of spinal cord; _pr._
commissure connecting the posterior nerve-roots; _ch._ notochord;
_x._ subnotochordal rod; _ao._ aorta; _sv._ sinus venosus; _cav._
cardinal vein; _ht._ heart; _pp._ body cavity; _pc._ pericardial
cavity; _oes._ solid oesophagus; _l._ liver; _mp._ muscle-plate;
_mp´._ inner layer of muscle-plate; _Vr._ rudiment of vertebral
body; _st._ segmental tube; _sd._ segmental duct; _sp.v._ spiral
valve; _v._ subintestinal vein.]

In the region of the head the mesoblast does not at first become
divided into somites; but on the formation of the gill clefts a
division takes place, which is apparently equivalent to the
segmentation of the mesoblast in the trunk. This division causes the
body cavity of the head to be divided up into a series of separate
segments, one of which is shewn in fig. 24, _pp_. The walls of the
segments eventually give rise to the main muscles of the branchial
clefts, and probably also to the muscles of the mandibular arch, of
the eye, and of other parts. The cephalic sections of the body cavity
will be spoken of as head cavities.

[FIG. 24. HORIZONTAL SECTION THROUGH THE LAST VISCERAL ARCH BUT ONE
OF AN EMBRYO OF PRISTIURUS.

_ep._ epiblast; _vc._ pouch of hypoblast which will form the walls
of a visceral cleft; _pp._ segment of body-cavity in visceral arch;
_aa._ aortic arch.]

In addition to the parts already mentioned the mesoblast gives rise to
the whole of the vascular system, and to the generative system. The
heart is formed from part of the splanchnic mesoblast, and the
generative system from a portion of the mesoblast of the dorsal part
of the body cavity.

The hypoblast. Very shortly after the formation of the mesoblastic
plates as lateral differentiations of the lower layer cells, an axial
differentiation of the hypoblast appears, which gives rise to the
notochord very much in the same way as in Amphioxus.

At first the hypoblast along the axial line forms a single layer in
contact with the epiblast. Along this line a rod-like thickening of
the hypoblast very soon appears (fig. 25, B and C, _Ch´_) at the head
end of the embryo, and gradually extends backwards. This is the
rudiment of the notochord; it remains attached for some time to the
hypoblast, and becomes separated from it first at the head end of the
embryo (fig. 25 A, _ch_): the separation is then carried backwards.

A series of sections taken through an embryo shortly after the first
differentiation of the notochord presents the following characters.

In the hindermost sections the hypoblast retains a perfectly normal
structure and uniform thickness throughout. In the next few sections
(fig. 25 C, _Ch´_) a slight thickening is to be observed in it,
immediately below the medullary groove. The layer, which elsewhere is
composed of a single row of cells, here becomes two cells deep, but no
sign of a division into two layers is exhibited.

In the next few sections the thickening of the hypoblast becomes much
more pronounced; we have, in fact, a ridge projecting from the
hypoblast towards the epiblast (fig. 25 B, _Ch´_). This ridge is
pressed firmly against the epiblast, and causes in it a slight
indentation. The hypoblast in the region of the ridge is formed of two
layers of cells, the ridge being entirely due to the uppermost of the
two.

[FIG. 25. THREE SECTIONS OF A PRISTIURUS EMBRYO SLIGHTLY OLDER THAN
FIG. 28 B.

The sections shew the development of the notochord.

_Ch._ notochord; _Ch´._ developing notochord; _mg._ medullary
groove; _lp._ lateral plate of mesoblast; _ep._ epiblast; _hy._
hypoblast.]

In sections in front of this a cylindrical rod, which can at once be
recognized as the notochord, and is continuous with the ridge just
described, begins to be split off from the hypoblast (fig. 25 A,
_Ch_). It is difficult to say at what point the separation of this rod
from the hypoblast is completed, since all intermediate gradations
between complete separation and complete attachment are to be seen.

Shortly after the separation takes place, a fairly thick bridge is
found connecting the two lateral halves of the hypoblast, but this
bridge is anteriorly excessively delicate and thin, and in some cases
is barely visible except with high powers. In some sections I have
observed possible indications of the process like that described by
Calberla for Petronyzon, by which the lateral parts of the hypoblast
grow in underneath the axial part, and so isolate it bodily as the
notochord.

It is not absolutely clear whether the notochord is to be regarded as
an axial differentiation of the hypoblast, or as an axial
differentiation of the lower layer cells.

The facts of development both in Amphioxus and Elasmobranchii tend
towards the former view; but the nearly simultaneous differentiation
of the notochord and the mesoblastic plates lends some support to the
supposition that the notochord may be merely a median plate of
mesoblast developed slightly later than the two lateral plates.

The alimentary canal or mesenteron was left as a space between the
hypoblast and the yolk, ending blindly in front, but opening behind by
a widish aperture, the blastopore or anus of Rusconi (_vide_ fig. 19
B).

[FIG. 26. SECTION THROUGH THE ANTERIOR PART OF A PRISTIURUS EMBRYO
TO SHEW THE FORMATION OF THE ALIMENTARY TRACT.

_Ch._ notochord; _hy._ hypoblast; _al._ alimentary tract; _na._
cells passing in from the yolk to form the ventral wall of the
alimentary tract.]

The conversion of this irregular cavity into a closed canal commences
first of all at the anterior extremity. In this conversion two
distinct processes are concerned. One of these is a process of folding
off of the embryo from the blastoderm. The other is a simple growth of
cells independent of any fold. To the first of these processes the
depth and narrowness of the alimentary cavity is due; the second is
concerned in forming its ventral wall. The process of the folding off
of the embryo from the blastoderm resembles exactly the similar
process in the embryo bird. The fold is a perfectly continuous one
round the front end of the embryo, but may be conveniently spoken of
as composed of a head-fold and two lateral folds.

Of far greater interest than the nature of these folds is the
formation of the ventral wall of the alimentary canal. This originates
in a growth of cells from the two sides to the middle line (fig. 26).
The cells for it are not however mainly derived from pre-existing
hypoblast cells, but are formed _de novo_ around the nuclei of the
yolk which have already been spoken of (fig. 26, _na_). The ventral
wall of the mesenteron is in fact, to a large extent at any rate,
formed as a differentiation of the primitive yolk floor.

[FIG. 27. LONGITUDINAL VERTICAL SECTION OF AN EMBRYO SLIGHTLY
YOUNGER THAN THAT IN FIG. 26 D.

The section shews the communication which exists between the neural
and alimentary canals.

_nc._ neural canal; _al._ alimentary tract; _Ch._ notochord; _Ts._
tail swelling.]

The folding off and closing of the alimentary canal in the anterior
part of the body proceeds rapidly, and not only is a considerable
tract of the alimentary canal formed, but a great part of the head is
completely folded off from the yolk before the medullary groove is
closed.

The posterior part of the alimentary canal retains for a longer time
its primitive condition. Finally however it also becomes closed in, by
the lips of the blastopore at the hind end of the embryo meeting and
uniting. The peculiarity of the closing in of the posterior part of
the alimentary canal consists in the fact that a similar continuity to
that in Amphioxus obtains between the neural and alimentary canals.
This is due to the medullary folds being continuous at the end of the
tail with the lips of the blastopore, which close in the hind end of
the alimentary canal; so that, when the medullary folds unite to form
a canal, this canal becomes continuous with the alimentary canal,
which is closed in at the same time. In other words, the medullary
folds assist in enveloping the blastopore which does not therefore
become absolutely closed, but opens into the floor of the neural
canal. It will afterwards be shewn that it is only the posterior part
of the blastopore that becomes closed during the above process, and
that the anterior and ventral part long remains open. The general
arrangement of the parts, at the time when the hind end of the
mesenteron is first closed, is shewn in fig. 27. The same points may
be seen in the diagrammatic longitudinal section fig. 19 C.

The middle portion of the alimentary tract is the last to be closed in
since it remains till late in embryonic life as the umbilical or
vitelline canal, connecting the yolk-sack with the alimentary cavity.
The umbilical canal falls into the alimentary tract immediately behind
the entrance of the hepatic duct.

[FIG. 27*. TRANSVERSE SECTION THROUGH THE TAIL REGION OF A
PRISTIURUS EMBRYO OF THE SAME AGE AS FIG. 28 E.

_df_. dorsal fin; _sp.c_. spinal cord; _pp_. body cavity; _sp_.
splanchnic layer of mesoblast; so. somatic layer of mesoblast; _mp_.
commencing differentiation of muscles; _ch_. notochord; _x_.
subnotochordal rod arising as an outgrowth of the dorsal wall of the
alimentary tract; _al_. alimentary tract.]

At a fairly early stage of development a rod is constricted off from
the dorsal wall of the alimentary canal (figs. 27* and 23 _x_), which
is known as the subnotochordal rod. It is placed immediately below the
notochord, and disappears during embryonic life.

_General features of the Elasmobranch embryo at successive stages._

Shortly after the three germinal layers become definitely established,
the rudiment of the embryo, as visible from the surface, consists of
an oblong plate, which extends inwards from the periphery of the
blastoderm, and is bounded on its inner side by a head-fold and two
lateral folds (fig. 28 B). This plate is the medullary plate; along
its axial line is a shallow groove--the medullary groove (_mg_). The
rudiment of the embryo rapidly increases in length, and takes a
spatula-like form (fig. 28 C). The front part of it, turned away from
the edge of the blastoderm, soon becomes dilated into a broad
plate,--the cephalic plate (_h_)--while the tail end at the edge of
the blastoderm is also enlarged, being formed of a pair of
swellings--the tail swellings (_ts_)--derived from the lateral parts
of the original embryonic rim. By this stage a certain number of
mesoblastic somites have become formed but are not shewn in my figure.
They are the foremost somites of the trunk, and those behind them
continue to be added, like the segments in Chætopods, between the last
formed somite and the end of the body. The increase in length of the
body mainly takes place by growth in the region between the last
mesoblastic somite and the end of the tail. The anterior part of the
body is now completely folded off from the blastoderm, and the
medullary groove of the earlier stage has become converted into a
closed canal.

By the next stage (fig. 28 D) the embryo has become so much folded off
from the yolk both in front and behind that the separate parts of it
begin to be easily recognizable.

The embryo is attached to the yolk by a distinct stalk or cord, which
in the succeeding stages gradually narrows and elongates, and is known
as the umbilical cord (_so. s_.). The medullary canal has now become
completely closed. The anterior region constitutes the brain; and in
this part slight constrictions, not perceptible in views of the embryo
as a transparent object, mark off three vesicles. These vesicles are
known as the fore, mid, and hind brain. From the fore-brain there is
an outgrowth on each side, the first rudiment of the optic vesicles
(_op_). The tail swellings are still conspicuous.

[FIG. 28. VIEWS OF ELASMOBRANCH EMBRYOS.
A-F. PRISTIURUS. G. and H. SCYLLIUM.

A. A blastoderm before the formation of the medullary plate. _sc._
segmentation cavity; _es._ embryonic swelling.
B. A somewhat older blastoderm in which the medullary groove has
been established. _mg._ medullary groove.
C. An embryo from the dorsal surface, as an opaque object, after the
medullary groove has become posteriorly converted into a tube. _mg._
medullary groove: the reference line points very nearly to the
junction between the open medullary groove with the medullary tube;
_h._ cephalic plate; _ts._ tail swelling.
D. Side view of a somewhat older embryo as a transparent object.
_ch._ notochord; _op._ optic vesicle; _1.v.c._ 1st visceral cleft;
_al._ alimentary tract; _so.s._ stalk connecting the yolk-sack with
the embryo.
E. Side view of an older embryo as a transparent object. _mp._
muscle-plates; _au.v._ auditory vesicle; _vc._ visceral cleft; _ht._
heart; _m._ mouth invagination; _an._ anal diverticulum; _al.v._
posterior vesicle of postanal gut.
F. G. H. Older embryos as opaque objects.]

The tissues of the body have now become fairly transparent, and there
may be seen at the sides of the body seventeen mesoblastic somites.
The notochord, which was formed long before the stage represented in
figure 28 D, is now also distinctly visible. It extends from almost
the extreme posterior to the anterior end of the embryo, and lies
between the ventral wall of the spinal canal and the dorsal wall of
the intestine. Round its posterior end the neural and alimentary
tracts become continuous with each other. Anteriorly the termination
of the notochord cannot be seen, it can only be traced into a mass of
mesoblast at the base of the brain, which there separates the epiblast
from the hypoblast. The alimentary canal (_al_) is completely closed
anteriorly and posteriorly, though still widely open to the yolk-sack
in the middle part of its course. In the region of the head it
exhibits on each side a slight bulging outwards, the rudiment of the
first visceral cleft. This is represented in the figure by two lines
(1. _v.c._).

The embryo represented in fig. 28 E is far larger than the one just
described, but it has not been convenient to represent this increase
of size in the figure. Accompanying this increase in size, the folding
off from the yolk has considerably progressed, and the stalk which
unites the embryo with the yolk is proportionately narrower and longer
than before.

The brain is now very distinctly divided into the three lobes, the
rudiments of which appeared during the last stage. From the foremost
of these the optic vesicles now present themselves as well-marked
lateral outgrowths, towards which there has appeared an involution
from the external skin (_op_) to form the lens.

A fresh organ of sense, the auditory sack, now for the first time
becomes visible as a shallow pit in the external skin on each side of
the hind-brain (_au.v_). The epiblast which is involuted to form this
pit becomes much thickened, and thereby the opacity, indicated in the
figure, is produced.

The mesoblastic somites have greatly increased in number by the
formation of fresh somites in the tail. Thirty-eight of them were
present in the embryo figured. The mesoblast at the base of the brain
is more bulky, and there is still a mass of unsegmented mesoblast
which forms the tail swellings. The first rudiment of the heart (_ht_)
becomes visible during this stage as a cavity between the mesoblast of
the splanchnopleure and the hypoblast.

The fore and hind guts are now longer than they were. An invagination
from the exterior to form the mouth has appeared (_m_) on the ventral
side of the head close to the base of the thalamencephalon. The upper
end of this eventually becomes constricted off as the pituitary body,
and an indication of the future position of the anus is afforded by a
slight diverticulum of the hind gut towards the exterior, some little
distance from the posterior end of the embryo (_an_). The portion of
the alimentary canal behind this point, though at this stage large,
and even dilated into a vesicle at its posterior end (_al.v_), becomes
eventually completely atrophied. It is known as the postanal gut. In
the region of the throat the rudiment of a second visceral cleft has
appeared behind the first; neither of them is as yet open to the
exterior.

In a somewhat older embryo the first spontaneous movements take place,
and consist in somewhat rapid excursions of the embryo from side to
side, produced by a serpentine motion of the body.

[FIG. 28*. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL OF AN
EMBRYO OF THE SAME AGE AS FIG. 28 F.

A is the posterior section.

_nc._ neural canal; _al._ postanal gut; _alv._ caudal vesicle of
postanal gut; _x._ subnotochord rod; _mp._ muscle-plate; _ch._
notochord; _cl.al._ cloaca; _ao._ aorta; _v.cau._ caudal vein.]

A ventral flexure of the præoral part of the head, known as the
cranial flexure, which commenced in earlier stages (fig. 28 D and E),
has now become very evident, and the mid-brain[19] begins to project
in the same manner as in the embryo fowl on the third day, and will
soon form the anterior termination of the long axis of the embryo. The
fore-brain has increased in size and distinctness, and the anterior
part of it may now be looked on as the unpaired rudiment of the
cerebral hemispheres.

[19] The part of the brain which I have here called mid-brain,
and which unquestionably corresponds to the part called mid-brain
in the embryos of higher vertebrates, becomes in the adult what
Miklucho-Maclay and Gegenbaur called the vesicle of the third
ventricle or thalamencephalon.

Further changes have taken place in the organs of sense, especially in
the eye, in which the involution for the lens has made considerable
progress. The number of the muscle-plates has again increased, but
there is still a region of unsegmented mesoblast in the tail. The
thickened portions of mesoblast, which caused the tail swellings, are
still to be seen, and would seem to act as the reserve from which is
drawn the matter for the rapid growth of the tail, which occurs soon
after this. The mass of the mesoblast at the base of the brain has
again increased. No fresh features of interest are to be seen in the
notochord. The heart is very much more conspicuous than before, and
its commencing flexure is very apparent. It now beats actively. The
postanal gut is much longer than during the last stage; and the point
where the anus will appear is very easily detected by a bulging out of
the gut towards the external skin. The alimentary vesicle at the end
of the postanal gut, first observable during the last stage, is now a
more conspicuous organ. There are three visceral clefts, none of which
are as yet open to the exterior.

Figure 28 F represents a considerably older embryo viewed as an opaque
object, and fig. 29 A is a view of the head as a transparent object.
The stalk connecting it with the yolk is now, comparatively speaking,
quite narrow, and is of sufficient length to permit the embryo to
execute considerable movements.

The tail has grown immensely, but is still dilated terminally. The
terminal dilatation is mainly due to the alimentary vesicle (fig. 28*
_alv_), but the postanal section of the alimentary tract in front of
this is now a solid cord of cells. Both the alimentary vesicle and
this cord very soon disappear. Their relations are shewn in section in
fig. 28*.

The two pairs of limbs have appeared as differentiations of a
continuous but not very conspicuous epiblastic thickening, which is
probably the rudiment of a lateral fin. The anterior pair is situated
just at the front end of the umbilical stalk; and the posterior pair,
which is the later developed and less conspicuous of the two, is
situated some little distance behind the stalk.

[FIG. 29. VIEWS OF THE HEAD OF ELASMOBRANCH EMBRYOS AT TWO STAGES AS
TRANSPARENT OBJECTS.

A. Pristiurus embryo of the same stage as fig. 28 F.
B. Somewhat older Scyllium embryo.

_III._ third nerve; _V._ fifth nerve; _VII._ seventh nerve; _au.n._
auditory nerve; _gl._ glossopharyngeal nerve; _Vg._ vagus nerve;
_fb._ fore-brain; _pn._ pineal gland; _mb._ mid-brain; _hb._
hind-brain; _iv.v._ fourth ventricle; _cb._ cerebellum; _ol._
olfactory pit; _op._ eye; _au.V._ auditory vesicle; _m._ mesoblast
at base of brain; _ch._ notochord; _ht._ heart; _Vc._ visceral
clefts; _eg._ external gills; _pp._ sections of body cavity in the
head.]

The cranial flexure has greatly increased, and the angle between the
long axis of the front part of the head and of the body is less than a
right angle. The conspicuous mid-brain (29 A, _mb_) forms the anterior
termination of the long axis of the body. The thin roof of the fourth
ventricle (_hb_) may be noticed in the figure behind the mid-brain.
The auditory sack (_au.V_) is nearly closed, and its opening is not
shewn in the figure. In the eye (_op_) the lens is completely formed.
The olfactory pit (_ol_) is seen a little in front of the eye.

Owing to the opacity of the embryo, the muscle-plates are only
indistinctly indicated in fig. 28 F, and no other features of the
mesoblast are to be seen.

The mouth is now a deep pit, the hind borders of which are almost
completely formed by a thickening in front of the first branchial or
visceral cleft, which may be called the first branchial arch or
mandibular arch.

Four branchial clefts are now visible, all of which are open to the
exterior, but in the embryo, viewed as a transparent object, two more,
not open to the exterior, are visible behind the last of these.

Between each of these and behind the last one there is a thickening of
the mesoblast which gives rise to a branchial arch. The arch between
the first and second cleft is known as the hyoid arch.

Fig. 29 B is a representation of the head of a slightly older embryo
in which papillæ may be seen in the front wall of the second, third,
and fourth branchial clefts; these papillæ are the commencements of
filiform processes which grow out from the gill-clefts and form
external gills. The peculiar ventral curvature of the anterior end of
the notochord (_ch_) both in this and in the preceding figure deserves
notice.

A peculiar feature in the anatomy makes its appearance at this period,
viz. the replacement of the original hollow oesophagus by a solid cord
of cells (fig. 23 A, _oes_) in which a lumen does not reappear till
very much later. I have found that in some Teleostei (the Salmon) long
after they are hatched a similar solidity in the oesophagus is
present. It appears not impossible that this feature in the oesophagus
may be connected with the fact that in the ancestors of the present
types the oesophagus was perforated by gill slits; and that in the
process of embryonic abbreviation the stage with the perforated
oesophagus became replaced by a stage with a cord of indifferent cells
(the oesophagus being in the embryo quite functionless) out of which
the non-perforated oesophagus was directly formed. In the higher types
the process of development appears to have become quite direct.

By this stage all the parts of the embryo have become established, and
in the succeeding stages the features characteristic of the genus and
species are gradually acquired.

Two embryos of Scyllium are represented in fig. 28 G and H, the head
and anterior part of the trunk being represented in fig. G, and the
whole embryo at a much later stage in fig. H.

In both of these, and especially in the second, an apparent diminution
of the cranial flexure is very marked. This diminution is due to the
increase in the size of the cerebral hemispheres, which grow upwards
and forwards, and press the original fore-brain against the mid-brain
behind.

In fig. G the rudiments of the nasal sacks are clearly visible as
small open pits.

The first cleft is no longer similar to the rest, but by the closure
of the lower part has commenced to be metamorphosed into the spiracle.

Accompanying the change in position of the first cleft, the mandibular
arch has begun to bend round so as to enclose the front as well as the
sides of the mouth. By this change in the mandibular arch the mouth
becomes narrowed in an antero-posterior direction.

In fig. H are seen the long filiform external gills which now project
out from all the visceral clefts, including the spiracle. They are
attached to the front wall of the spiracle, to both walls of the next
four clefts, and to the front wall of the last cleft. They have very
possibly become specially developed to facilitate respiration within
the egg; and they disappear before the close of larval life.

When the young of Scyllium and other Sharks are hatched they have all
the external characters of the adult. In Raja and Torpedo the early
stages, up to the acquirement of a shark-like form, are similar to
those in the Selachoidei, but during the later embryonic stages the
body gradually flattens out, and assumes the adult form, which is thus
clearly shewn to be a secondary acquirement.

An embryonic gill cleft behind the last present in the adult is found
(Wyman, No. 54) in the embryo of Raja batis.

The unpaired fins are developed in Elasmobranchs as a fold of skin on
the dorsal side, which is continued round the end of the tail along
the ventral side to the anus. Local developments of this give rise to
the dorsal and anal fins. The caudal fin is at first symmetrical, but
a special lower lobe grows out and gives to it a heterocercal
character.

_Enclosure of the yolk-sack and its relation to the embryo._

The blastoderm at the stage represented in fig. 28 A and B forms a
small and nearly circular patch on the surface of the yolk, composed
of epiblast and lower layer cells. While the body of the embryo is
gradually being moulded this patch grows till it envelopes the yolk;
the growth is not uniform, but is less rapid in the immediate
neighbourhood of the embryonic part of the blastoderm than elsewhere.
As a consequence of this, that part of the edge, to which the embryo
is attached, forms a bay in the otherwise regular outline of the edge
of the blastoderm, and by the time that about two-thirds of the yolk
is enclosed this bay is very conspicuous. It is shewn in fig. 30 A,
where _bl_ points to the blastoderm, and _yk_ to the part of the yolk
not yet covered by the blastoderm. The embryo at this time is only
connected with the yolk-sack by a narrow umbilical cord; but, as shewn
in the figure, is still attached to the edge of the blastoderm.

[FIG. 30. THREE VIEWS OF THE VITELLUS OF AN ELASMOBRANCH, SHEWING
THE EMBRYO, THE BLASTODERM, AND THE VESSELS OF THE YOLK-SACK.

The shaded part (_bl_) is the blastoderm; the white part the
uncovered yolk.

A. Young stage with the embryo still attached at the edge of the
blastoderm.
B. Older stage with the yolk not quite enclosed by the blastoderm.
C. Stage after the complete enclosure of the yolk.

_yk._ yolk; _bl._ blastoderm; _v._ venous trunks of yolk-sack; _a._
arterial trunks of yolk-sack; _y._ point of closure of the yolk
blastopore; _x._ portion of the blastoderm outside the arterial
sinus terminalis.]

Shortly subsequent to this the bay in the blastoderm, at the head of
which the embryo is attached, becomes obliterated by its two sides
coming together and coalescing. The embryo then ceases to be attached
at the edge of the blastoderm. But a linear streak formed by the
coalesced edges of the blastoderm is left connecting the embryo with
the edge of the blastoderm. This streak is probably analogous to
(though not genetically related with) the primitive streak in the
Amniota.

This stage is represented in fig. 30 B. In this figure there is only a
small patch of yolk (_yk_) not yet enclosed, which is situated at some
little distance behind the embryo. Throughout all this period the edge
of the blastoderm has remained thickened: a feature which persists
till the complete investment of the yolk, which takes place shortly
after the stage last described. In this thickened edge a circular vein
arises which brings back the blood from the yolk-sack to the embryo.
The opening in the blastoderm, exposing the portion of the yolk not
yet covered, may be conveniently called the yolk blastopore. It is
interesting to notice that, owing to the large size of the yolk in
Elasmobranchs, the posterior part of the primitive blastopore becomes
encircled by the medullary folds and tail swellings, and is so closed
long before the anterior and more ventral part, which is represented
by the uncovered portion of the yolk. It is also worth remarking that,
owing to the embryo becoming removed from the edge of the blastoderm,
the final closure of the yolk blastopore takes place _at some little
distance from the embryo_.

The blastoderm enclosing the yolk is formed of an external layer of
epiblast, a layer of mesoblast below in which the blood-vessels are
developed, and within this a layer of hypoblast, which is especially
well marked and ciliated (Leydig, No. 46) in the umbilical stalk,
where it lines the canal leading from the yolk-sack to the intestine.
In the region of the yolk-sack proper the blastoderm is so thin that
it is not easy to be quite sure that a layer of hypoblast is
throughout distinct. Both the hypoblast and mesoblast of the yolk-sack
are formed by a differentiation of the primitive lower layer cells.

Nutriment from the yolk-sack is brought to the embryo partly through
the umbilical canal and so into the intestine, and partly by means of
blood-vessels in the mesoblast of the sack. The blood-vessels arise
before the blastoderm has completely covered the yolk.

Fig. 30 A represents the earliest stage of the circulation of the
yolk-sack. At this stage there is visible a single arterial trunk
(_a_) passing forwards from the embryo and dividing into two branches.
No venous trunk could be detected with the simple microscope, but
probably venous channels were present in the thickened edge of the
blastoderm.

In fig. 30 B the circulation is greatly advanced. The blastoderm has
now nearly completely enveloped the yolk, and there remains only a
small circular space (_yk_) not enclosed by it. The arterial trunk is
present as before, and divides in front of the embryo into two
branches which turn backwards and form a nearly complete ring round
the embryo. In general appearance this ring resembles the sinus
terminalis of the area vasculosa of the Bird, but in reality bears
quite a different relation to the circulation. It gives off branches
on its inner side only.

A venous system of returning vessels is now fully developed, and its
relations are very remarkable. There is a main venous ring in the
thickened edge of the blastoderm, which is connected with the embryo
by a single stem running along the seam where the edges of the
blastoderm have coalesced. Since the venous trunks are only developed
behind the embryo, it is only the posterior part of the arterial ring
that gives off branches.

The succeeding stage (fig. 30 C) is also one of considerable interest.
The arterial ring has greatly extended, and now embraces nearly half
the yolk, and sends off trunks on its inner side along its whole
circumference. More important changes have taken place in the venous
system. The blastoderm has now completely enveloped the yolk, and the
venous ring is therefore reduced to a point. The small veins which
originally started from it may be observed diverging in a brush-like
fashion from the termination of the unpaired trunk, which originally
connected the venous ring with the heart.

At a still later stage the arterial ring embraces the whole yolk, and,
as a result of this, vanishes in its turn, as did the venous ring
before it. There is then present a single arterial and a single venous
trunk. The arterial trunk is a branch of the dorsal aorta, and the
venous trunk originally falls into the heart together with the
subintestinal or splanchnic vein. On the formation of the liver the
proximal end of the subintestinal vein becomes the portal vein, and it
is joined just as it enters the liver by the venous trunk from the
yolk-sack. The venous trunk leaves the body on the right side, and the
arterial on the left.

The yolk-sack persists during the whole of embryonic life, and in the
majority of Elasmobranch embryos there arises within the body walls an
outgrowth from the umbilical canal into which a large amount of the
yolk passes. This outgrowth forms an internal yolk-sack. In Mustelus
vulgaris the internal yolk-sack is very small, and in Mustelus lævis
it is absent. The latter species, which is one of those in which
development takes place within the uterus, presents a remarkable
peculiarity in that the vascular surface of the yolk-sack becomes
raised into a number of folds, which fit into corresponding
depressions in the vascular walls of the uterus. The yolk-sack becomes
in this way firmly attached to the walls of the uterus, and the two
together constitute a kind of placenta. A similar placenta is found in
Carcharias.

After the embryo is hatched or born, as the case may be, the yolk-sack
becomes rapidly absorbed.

BIBLIOGRAPHY.

(40) F. M. Balfour. "A preliminary account of the development of the
Elasmobranch Fishes." _Quart. J. of Micr. Science_, Vol. XIV. 1876.

(41) F. M. Balfour. "A Monograph on the development of Elasmobranch
Fishes." London, 1878. Reprinted from the _Journal of Anat. and
Physiol._ for 1876, 1877, and 1878.

(42) Z. Gerbe. "_Recherches sur la segmentation de la cicatrule et la
formation des produits adventifs de l'oeuf des Plagiostomes et
particulièrement des Raies._" Vide also _Journal de l'Anatomie et de
la Physiologie_, 1872.

(43) W. His. "Ueb. d. Bildung v. Haifischenembryonen." _Zeit. für
Anat. u. Entwick._, Vol. II. 1877.

(44) A. Kowalevsky. "Development of Acanthias vulgaris and Mustelus
lævis." (_Russian._) _Transactions of the Kiew Society of
Naturalists_, Vol. I. 1870.

(45) R. Leuckart. "Ueber die allmählige Bildung d. Körpergestalt bei
d. Rochen." _Zeit. f. wiss. Zool._, Bd. II., p. 258.

(46) Fr. Leydig. _Rochen u. Haie._ Leipzig, 1852.

(47) A. W. Malm. "Bidrag till kännedom om utvecklingen af Rajæ."
_Kongl. vetenskaps akademiens förhandlingar._ Stockholm, 1876.

(48) Joh. Müller. _Glatter Haie des Aristoteles und über die
Verschiedenheiten unter den Haifischen und Rochen in der Entwicklung
des Eies._ Berlin, 1840.

(49) S. L. Schenk. "Die Eier von Raja quadrimaculata innerhalb der
Eileiter." _Sitz. der k. Akad. Wien_, Vol. LXXIII. 1873.

(50) Alex. Schultz. "Zur Entwicklungsgeschichte des Selachiereies."
_Archiv für micro. Anat._, Vol. XI. 1875.

(51) Alex. Schultz. "Beitrag zur Entwicklungsgeschichte d.
Knorpelfische." _Archiv für micro. Anat._, Vol. XIII. 1877.

(52) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u.
Wirbellosen." _Arbeit. a. d. zool.-zoot. Instit. Würzburg_, Vol. II.
1875.

(53) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." _Arbeit.
a. d. zool.-zoot. Instit. Würzburg_, Vol. II. 1875.

(54) Wyman. "Observations on the Development of Raja batis." _Memoirs
of the American Academy of Arts and Sciences_, Vol. IX. 1864.

CHAPTER IV.

TELEOSTEI.

The majority of the Teleostei deposit their eggs before impregnation,
but some forms are viviparous, _e.g._ Blennius viviparus. Not a few
carry their eggs about; but this operation is with a few exceptions
performed by the male. In Syngnathus the eggs are carried in a
brood-pouch of the male situated behind the anus. Amongst the
Siluroids the male sometimes carries the eggs in the throat above the
gill clefts. Ostegeniosus militaris, Arius falcarius, and Arius fissus
have this peculiar habit.

The ovum when laid is usually invested in the zona radiata only,
though a vitelline membrane is sometimes present in addition, _e.g._
in the Herring. It is in most cases formed of a central yolk mass,
which may either be composed of a single large vitelline sphere, or of
distinct yolk spherules. The yolk mass is usually invested by a
granular protoplasmic layer, which is especially thickened at one pole
to form the germinal disc.

In the Herring's ovum the germinal disc is formed, as in many
Crustacea, at impregnation; the protoplasm which was previously
diffused through the egg becoming aggregated at the germinal pole and
round the periphery.

Impregnation is external, and on its occurrence a contraction of the
vitellus takes place, so that a space is formed between the vitellus
and the zona radiata, which becomes filled with fluid.

The peculiarities in the development of the Teleostean ovum can best
be understood by regarding it as an Elasmobranch ovum very much
reduced in size. It seems in fact very probable that the Teleostei are
in reality derived from a type of Fish with a much larger ovum. The
occurrence of a meroblastic segmentation, in spite of the ovum being
usually smaller than that of Amphibia and Acipenser, etc., in which
the segmentation is complete, as well as the solid origin of many of
the organs, receives its most plausible explanation on this
hypothesis.

The proportion of the germinal disc to the whole ovum varies
considerably. In very small eggs, such as those of the Herring, the
disc may form as much as a fifth of the whole.

The segmentation, which is preceded by active movements of the
germinal disc, is meroblastic. There is nothing very special to note
with reference to its general features, but while in large ova like
those of the Salmon the first furrows only penetrate for a certain
depth through the germinal disc, in small ova like those of the
Herring they extend through the whole thickness of the disc. During
the segmentation a great increase in the bulk of the blastoderm takes
place.

In hardened specimens a small cavity amongst the segmentation spheres
may be present at any early stage; but it is probably an artificial
product, and in any case has nothing to do with the true segmentation
cavity, which does not appear till near the close of segmentation. The
peripheral layer of granular matter, continuous with the germinal
disc, does not undergo division, but it becomes during the
segmentation specially thickened and then spreads itself under the
edge of the blastoderm; and, while remaining thicker in this region,
gradually grows inwards so as to form a continuous sub-blastodermic
layer. In this layer nuclei appear, which are equivalent to those in
the Elasmobranch ovum. A considerable number of these nuclei often
become visible simultaneously (van Beneden, No. 60) and they are
usually believed to arise spontaneously, though this is still
doubtful[20]. Around these nuclei portions of protoplasm are segmented
off, and cells are thus formed, which enter the blastoderm, and have
nearly the same destination as the homologous cells of the
Elasmobranch ovum.

[20] _Vide_ Vol. II. p. 108.

During the later stages of segmentation one end of the blastoderm
becomes thickened and forms the embryonic swelling; and a cavity
appears between the blastoderm and the yolk which is excentrically
situated near the non-embryonic part of the blastoderm. This cavity is
the true segmentation cavity. Both the cavity and the embryonic
swelling are seen in section in fig. 31 A and B.

In Leuciscus rutilus Bambeke describes a cavity as appearing in the
middle of the blastoderm during the later stages of segmentation. From
his figures it might be supposed that this cavity was equivalent to
the segmentation cavity of Elasmobranchs in its earliest condition,
but Bambeke states that it disappears and that it has no connection
with the true segmentation cavity. Bambeke and other investigators
have failed to recognize the homology of the segmentation cavity in
Teleostei with that in Elasmobranchii, Amphibia, etc.

With the appearance of the segmentation cavity the portion of the
blastoderm which forms its roof becomes thinned out, so that the whole
blastoderm consists of (1) a thickened edge especially prominent at
one point where it forms the embryonic swelling, and (2) a thinner
central portion. The changes which now take place result in the
differentiation of the embryonic layers, and in the rapid extension of
the blastoderm round the yolk, accompanied by a diminution in its
thickness.

[FIG. 31. LONGITUDINAL SECTIONS THROUGH THE BLASTODERM OF THE TROUT
AT AN EARLY STAGE OF DEVELOPMENT.

A. at the close of the segmentation; B. after the differentiation of
the germinal layers.

_ep´._ epidermic layer of the epiblast; _sc._ segmentation cavity.]

The first differentiation of the layers consists in a single row of
cells on the surface of the blastoderm becoming distinctly marked off
as a special layer (fig. 31 A); which however does not constitute the
whole epiblast but only a small part of it, which will be spoken of as
the epidermic layer. The complete differentiation of the epiblast is
effected by the cells of the thickened edge of the blastoderm becoming
divided into two strata (fig. 31 B). The upper stratum constitutes the
epiblast. It is divided into two layers, viz., the external epidermic
layer already mentioned, and an internal layer known as the nervous
layer, formed of several rows of vertically arranged cells. According
to the unanimous testimony of investigators the roof of the
segmentation cavity is formed of epiblast cells only. The lower
stratum in the thickened rim of the blastoderm is several rows of
cells deep, and corresponds with the lower layer cells or primitive
hypoblast in Elasmobranchii. It is continuous at the edge of the
blastoderm with the nervous layer of the epiblast.

In smaller Teleostean eggs there is formed, before the blastoderm
becomes differentiated into epiblast and lower layer cells, a complete
stratum of cells around the nuclei in the granular layer underneath
the blastoderm. This layer is the hypoblast; and in these forms the
lower layer cells of the blastoderm are stated to become converted
into mesoblast only. In the larger Teleostean eggs, such as those of
the Salmonidæ, the hypoblast, as in Elasmobranchs, appears to be only
partially formed from the nuclei of the granular layer. In these forms
however, as in the smaller Teleostean ova and in Elasmobranchii, the
cells derived from the granular stratum give rise to a more or less
complete cellular floor for the segmentation cavity. The segmentation
cavity thus becomes enclosed between an hypoblastic floor and an
epiblastic roof several cells deep. It becomes obliterated shortly
after the appearance of the medullary plate.

At about the time when the three layers become established the
embryonic swelling takes a somewhat shield-like form (fig. 33 A).
Posteriorly it terminates in a caudal prominence (_ts_) homologous
with the pair of caudal swellings in Elasmobranchs. The homologue of
the medullary groove very soon appears as a shallow groove along the
axial line of the shield. After these changes there takes place in the
embryonic layers a series of differentiations leading to the
establishment of the definite organs. These changes are much more
difficult to follow in the Teleostei than in the Elasmobranchii, owing
partly to the similarity of the cells of the various layers, and
partly to the primitive solidity of all the organs.

The first changes in the epiblast give rise to the central nervous
system. The epiblast, consisting of the nervous and epidermic strata
already indicated, becomes thickened along the axis of the embryo and
forms a keel projecting towards the yolk below: so great is the size
of this keel in the front part of the embryo that it influences the
form of the whole body and causes the outline of the surface adjoining
the yolk to form a strong ridge moulded on the keel of the epiblast
(fig. 32 A and B). Along the dorsal line of the epiblast keel is
placed the shallow medullary groove; and according to Calberla (No.
61) the keel is formed by the folding together of the two sides of the
primitively uniform epiblastic layer. The keel becomes gradually
constricted off from the external epiblast and then forms a _solid
cord_ below it. Subsequently there appears in this cord a median
slit-like canal, which forms the permanent central canal of the
cerebrospinal cord. The peculiarity in the formation of the central
nervous system of Teleostei consists in the fact that it is not formed
by the folding over of the sides of the medullary groove into a canal,
but by the separation, below the medullary groove, of a solid cord of
epiblast in which the central canal is subsequently formed. Various
views have been put forward to explain the apparently startling
difference between Teleostei, with which Lepidosteus and Petromyzon
agree, and other vertebrate forms. The explanations of Götte and
Calberla appear to me to contain between them the truth in this
matter. The groove above in part represents the medullary groove; but
the closure of the groove is represented by the folding together of
the lateral parts of the epiblast plate to form the medullary keel.

According to Götte this is the whole explanation, but Calberla states
for Syngnathus and Salmo that the epidermic layer of the epiblast is
carried down into the keel as a double layer just as if it had been
really folded in. This ingrowth of the epidermic layer is shewn in
fig. 32 A where it is just commencing to pass into the keel; and at a
later stage in fig. 32 B where the keel has reached its greatest
depth.

Götte maintains that Calberla's statements are not to be trusted, and
I have myself been unable to confirm them for Teleostei or
Lepidosteus; but if they could be accepted the difference in the
formation of the medullary canal in Teleostei and in other Vertebrata
would become altogether unimportant and consist simply in the fact
that the ordinary open medullary groove is in Teleostei obliterated in
its inner part by the two sides of the groove coming together. Both
layers of epiblast would thus have a share in the formation of the
central nervous system; the epidermic layer giving rise to the lining
epithelial cells of the central canal, and the nervous layer to the
true nervous tissue.

The separation of the solid nervous system from the epiblast takes
place relatively very late; and, before it has been completed, the
first traces of the auditory pits, of the optic vesicles, and of the
olfactory pits are visible. The auditory pit arises as a solid
thickening of the nervous layer of the epiblast at its point of
junction with the medullary keel; and the optic vesicles spring as
solid outgrowths from part of the keel itself. The olfactory pits are
barely indicated as thickenings of the nervous layer of the epiblast.

[FIG. 32. TWO TRANSVERSE SECTIONS OF SYNGNATHUS. (After Calberla.)

A. Younger stage before the definite establishment of the notochord.
B. Older stage.
The epidermic layer of the epiblast is represented in black.

_ep._ epidermic layer of epiblast; _mc._ neural cord; _hy._
hypoblast; _me._ mesoblast; _ch._ notochord.]

At this early stage all the organs of special sense are attached to a
layer continuous with or forming part of the central nervous system;
and this fact has led Götte (No. 63) to speak of a special-sense
plate, belonging to the central nervous system and not to the skin,
from which all the organs of special sense are developed; and to
conclude that a serial homology exists between these organs in their
development. A comparison between Teleostei and other forms shews that
this view cannot be upheld; even in Teleostei the auditory and
olfactory rudiments arise rather from the epiblast at the sides of the
brain than from the brain itself, while the optic vesicles spring from
the first directly from the medullary keel, and are therefore
connected with the central nervous system rather than with the
external epiblast. In a slightly later stage the different connections
of the two sets of sense organs is conclusively shewn by the fact
that, on the separation of the central nervous system from the
epiblast, the optic vesicles remain attached to the former, while the
auditory and olfactory vesicles are continuous with the latter.

After its separation from the central nervous system the remainder of
the epiblast gives rise to the skin, etc., and most probably the
epidermic stratum develops into the outer layer of the epidermis and
the nervous stratum into the mucous layer. The parts of the organs of
special sense, which arise from the epiblast, are developed from the
nervous layer. In the Trout (Oellacher, No. 72) both layers are
continued over the yolk-sack; but in Clupeus and Gasterosteus only the
epidermic has this extension. According to Götte the distinction
between the two layers becomes lost in the later embryonic stages.

Although it is thoroughly established that the mesoblast originates
from the lower of the two layers of the thickened embryonic rim, it is
nevertheless not quite certain whether it is a continuous layer
between the epiblast and hypoblast, or whether it forms two lateral
masses as in Elasmobranchs. The majority of observers take the former
view, while Calberla is inclined to adopt the latter. In the median
line of the embryo underneath the medullary groove there are
undoubtedly from the first certain cells which eventually give rise to
the notochord; and it is these cells the nature of which is in doubt.
They are certainly at first very indistinctly separated from the
mesoblast on the two sides, and Calberla also finds that there is no
sharp line separating them from the secondary hypoblast (fig. 32 A).
Whatever may be the origin of the notochord the mesoblast very soon
forms two lateral plates, one on each side of the body, and between
them is placed the notochord (fig. 32 B). The general fate of the two
mesoblast plates is the same as in Elasmobranchs. They are at first
quite solid and exhibit relatively late a division into splanchnic and
somatic layers, between which is placed the primitive body cavity. The
dorsal part of the plates becomes transversely segmented in the region
of the trunk; and thus gives rise to the mesoblastic somites, from
which the muscle plates and the perichordal parts of the vertebral
column are developed. The ventral or outer part remains unsegmented.
The cavity of the ventral section becomes the permanent body cavity.
It is continued forward into the head (Oellacher), and part of it
becomes separated off from the remainder as the pericardial cavity.

The hypoblast forms a continuous layer below the mesoblast, and, in
harmony with the generally confined character of the development of
the organs in Teleostei, there is no space left between it and the
yolk to represent the primitive alimentary cavity. The details of the
formation of the true alimentary tube have not been made out; it is
not however formed by a folding in of the lateral parts of the
hypoblast, but arises as a solid or nearly solid cord in the axial
line, between the notochord and the yolk, in which a lumen is
gradually established.

In the just hatched larva of an undetermined freshwater fish with a
very small yolk-sack I found that the yolk extended along the ventral
side of the embryo from almost the mouth to the end of the gut. The
gut had, except in the hinder part, the form of a solid cord resting
in a concavity of the yolk. In the hinder part of the gut a lumen was
present, and below this part the amount of yolk was small and the yolk
nuclei numerous. Near the limit of its posterior extension the yolk
broke up into a mass of cells, and the gut ended behind by falling
into this mass. These incomplete observations appear to shew that the
solid gut owes its origin in a large measure to nuclei derived from
the yolk.

When the yolk has become completely enveloped a postanal section of
gut undoubtedly becomes formed; and although, owing to the solid
condition of the central nervous system, a communication between the
neural and alimentary canals cannot at first take place, yet the
terminal vesicle of the postanal gut of Elasmobranchii is represented
by a large vesicle, originally discovered by Kupffer (No. 68), which
can easily be seen in the embryos of most Teleostei, but the relations
of which have not been satisfactorily worked out (_vide_ fig. 34,
_hyv_). As the tail end of the embryo becomes separated off from the
yolk the postanal vesicle atrophies.

General development of the Embryo. Attention has already been called
to the fact that the embryo first appears as a thickening of the edge
of the blastoderm which soon assumes a somewhat shield-like form (fig.
33 A). The hinder end of the embryo, which is placed at the edge of
the blastoderm, is somewhat prominent, and forms the caudal swelling
(_ts_). The axis of the embryo is marked by a shallow groove.

[FIG. 33. THREE STAGES IN THE DEVELOPMENT OF THE SALMON. (After
His.)

_ts._ tail swelling; _au.v._ auditory vesicle; _oc._ optic vesicle;
_ce._ cerebral rudiment; _m.b._ mid-brain; _cb._ cerebellum; _md._
medulla oblongata; _m.so._ mesoblastic somite.]

The body now rapidly elongates, and at the same time becomes
considerably narrower, while the groove along the axis becomes
shallower and disappears. The anterior, and at first proportionately a
very large part, soon becomes distinguished as the cephalic region
(fig. 33 B). The medullary cord in this region becomes very early
divided into three indistinctly separated lobes, representing the
fore, the mid, and the hind brains: of these the anterior is the
smallest. With it are connected the optic vesicles (_oc_)--solid at
first--which are pushed back into the region of the mid-brain.

The trunk grows in the usual way by the addition of fresh somites
behind.

After the yolk has become completely enveloped by the blastoderm the
tail becomes folded off, and the same process takes place at the front
end of the embryo. The free tail end of the embryo continues to grow,
remaining however closely applied to the yolk-sack, round which it
curls itself to an extent varying with the species (_vide_ fig. 34).

The general growth of the embryo during the later stages presents a
few special features of interest. The head is remarkable for the small
apparent amount of the cranial flexure. This is probably due to the
late development of the cerebral hemispheres. The flexure of the floor
of the brain is however quite as considerable in the Teleostei as in
other types. The gill clefts develop from before backwards. The first
cleft is the hyomandibular, and behind this there are the hyobranchial
and four branchial clefts. Simultaneously with the clefts there are
developed the branchial arches. The postoral arches formed are the
mandibular, hyoid and five branchial arches. In the case of the Salmon
all of these appear before hatching.

[FIG. 34. VIEW OF AN ADVANCED EMBRYO OF A HERRING IN THE EGG. (After
Kupffer.)

_oc._ eye; _ht._ heart; _hyv._ postanal vesicle; _ch._ notochord.]

The first cleft closes up very early (about the time of hatching in
the Salmon); and about the same time there springs a membranous fold
from the hyoid arch, which gradually grows backwards over the arches
following, and gives rise to the operculum. There appear in the Salmon
shortly before hatching double rows of papillæ on the four anterior
arches behind the hyoid. They are the rudiments of the branchiæ. They
reach a considerable length before they are covered in by the
opercular membrane. In Cobitis (Götte, No. 64) they appear in young
larvæ as filiform processes equivalent to the external gills of
Elasmobranchs. The extremities of these processes atrophy; while the
basal portions became the permanent gill lamellæ. The general relation
of the clefts, after the closure of the hyomandibular, is shewn in
fig. 35.

The air-bladder is formed as a dorsal outgrowth of the alimentary
tract very slightly in front of the liver. It grows in between the two
limbs of the mesentery, in which it extends itself backwards. It
appears in the Salmon, Carp, and other types to originate rather on
the right side of the median dorsal line, but whether this fact has
any special significance is rather doubtful. In the Salmon and Trout
it is formed considerably later than the liver, but the two are stated
by Von Baer to arise in the Carp nearly at the same time. The absence
of a pneumatic duct in the Physoclisti is due to a post-larval
atrophy. The region of the stomach is reduced almost to nothing in the
larva.

The oesophagus becomes solid, like that of Elasmobranchs, and remains
so for a considerable period after hatching.

The liver, in the earliest stage in which I have met with it in the
Trout (27 days after impregnation), is a solid ventral diverticulum of
the intestine, which in the region of the liver is itself without a
lumen.

[FIG. 35. DIAGRAMMATIC VIEW OF THE HEAD OF AN EMBRYO TELEOSTEAN,
WITH THE PRIMITIVE VASCULAR TRUNKS. (From Gegenbaur.)

_a._ auricle; _v._ ventricle; _abr._ branchial artery; _c´._
carotid; _ad._ aorta; _s._ branchial clefts; _sv._ sinus venosus;
_dc._ ductus Cuvieri; _n._ nasal pit.]

The excretory system commences with the formation of a segmental duct,
formed by a constriction of the parietal wall of the peritoneal
cavity. The anterior end remains open to the body cavity, and forms a
pronephros (head kidney). On the inner side of and opposite this
opening a glomerulus is developed, and the part of the body cavity
containing both the glomerulus and the opening of the pronephros
becomes shut off from the remainder of the body cavity, and forms a
completely closed Malpighian capsule.

The mesonephros (Wolffian body) is late in developing.

The unpaired fins arise as simple folds of the skin along the dorsal
and ventral edges, continuous with each other round the end of the
tail. The ventral fold ends anteriorly at the anus.

The dorsal and anal fins are developed from this fold by local
hypertrophy. The caudal fin[21], however, undergoes a more complicated
metamorphosis. It is at first symmetrical or nearly so on the dorsal
and ventral sides of the hinder end of the notochord. This symmetry is
not long retained, but very soon the ventral part of the fin with its
fin rays becomes much more developed than the dorsal part, and at the
same time the posterior part of the notochord bends up towards the
dorsal side.

[21] In addition to the paper by Alex. Agassiz (No. 55) _vide_
papers by Huxley, Kölliker, Vogt, etc.

In some few cases, _e.g._ Gadus, Salmo, owing to the simultaneous
appearance of a number of fin rays on the dorsal and ventral side of
the notochord the external symmetry of the tail is not interfered with
in the above processes. In most instances this is far from being the
case.

[FIG. 36. THREE STAGES IN THE DEVELOPMENT OF THE TAIL OF THE
FLOUNDER (PLEURONECTES). (After Agassiz.)

A. Stage in which the permanent caudal fin has commenced to be
visible as an enlargement of the ventral side of the embryonic
caudal fin.
B. Ganoid-like stage in which there is a true external heterocercal
tail.
C. Stage in which the embryonic caudal fin has almost completely
atrophied.

_c._ embryonic caudal fin; _f._ permanent caudal fin; _n._
notochord; _u._ urostyle.]

In the Flounder, which may serve as a type, the primitive symmetry is
very soon destroyed by the appearance of fin rays on the ventral side.
The region where they are present soon forms a lobe; and an externally
heterocercal tail is produced (fig. 36 A). The ventral lobe with its
rays continues to grow more prominent and causes the tail fin to
become bilobed (fig. 36 B); there being a dorsal embryonic lobe
without fin rays (_c_), which contains the notochord, and a ventral
lobe with fin rays, which will form the permanent caudal fin. In this
condition the tail fin resembles the usual Elasmobranch form or still
more that of some Ganoids, _e.g._ the Sturgeon. The ventral lobe
continues to develop; and soon projects beyond the dorsal, which
gradually atrophies together with the notochord contained in it, and
finally disappears, leaving hardly a trace on the dorsal side of the
tail (fig. 36 C, _c_). In the meantime the fin rays of the ventral
lobe gradually become parallel to the axis of the body; and this lobe,
together with a few accessory dorsal and ventral fin rays supported by
neural and hæmal processes, forms the permanent tail fin, which though
internally unsymmetrical, assumes an externally symmetrical form. The
upturned end of the notochord which was originally continued into the
primitive dorsal lobe becomes ensheathed in a bone without a division
into separate vertebræ. This bone forms the urostyle (_u_). The hæmal
processes belonging to it are represented by two cartilaginous masses,
which subsequently ossify, forming the hypural bones, and supporting
the primary fin rays of the tail (fig. 36 C). The ultimate changes of
the notochord and urostyle vary very considerably in the different
types of Teleostei. Teleostei may fairly be described as passing
through an Elasmobranch stage or a stage like that of most
pre-jurassic Ganoids or the Sturgeon as far as concerns their caudal
fin.

The anterior paired fins arise before the posterior; and there do not
appear to be any such indications as in Elasmobranchii of the paired
fins arising as parts of a continuous lateral fin.

Most osseous fishes pass through more or less considerable
post-embryonic changes, the most remarkable of which are those
undergone by the Pleuronectidæ[22]. These fishes, which in the adult
state have the eyes unsymmetrically placed on one side of the head,
leave the egg like normal Teleostei. In the majority of cases as they
become older the eye on the side, which in the adult is without an
eye, travels a little forward and then gradually rotates over the
dorsal side of the head, till finally it comes to lie on the same side
as the other eye. During this process the rotating eye always remains
at the surface and continues functional; and on the two eyes coming to
the same side of the head the side of the body without an organ of
vision loses its pigment cells, and becomes colourless.

[22] _Vide_ Agassiz (No. 56) and Steenstrup, Malm.

The dorsal fin, after the rotation of the eye, grows forward beyond
the level of the eyes. In the genus Plagusia (Steenstrup, Agassiz, No.
56) the dorsal fin grows forward before the rotation of the eye (the
right eye in this form), and causes some modifications in the process.
The eye in travelling round gradually sinks into the tissues of the
head, at the base of the fin above the frontal bone; and in this
process the original large opening of the orbit becomes much reduced.
Soon a fresh opening on the opposite and left side of the dorsal fin
is formed; so that the orbit has two external openings, one on the
left and one on the right side. The original one on the right soon
atrophies, and the eye passes through the tissues at the base of the
dorsal fin completely to the left side.

The rotating eye may be either the right or the left according to the
species.

The most remarkable feature in which the young of a large number of
Teleostei differ from the adults is the possession of provisional
spines, very often formed as osseous spinous projections the spaces
between which become filled up in the adult. These processes are
probably, as suggested by Günther, secondary developments acquired,
like the Zooea spines of larval Crustaceans, for purposes of defence.

The yolk-sack varies greatly in size in the different types of
Teleostei.

According as it is enclosed within the body-wall, or forms a distinct
ventral appendage, it is spoken of by Von Baer as an internal or
external yolk-sack. By Von Baer the yolk-sack is stated to remain in
communication with the intestine immediately behind the liver, while
Lereboullet states that there is a vitelline pedicle opening between
the stomach and the liver which persists till the absorption of the
yolk-sack. My own observations do not fully confirm either of these
statements for the Salmon and Trout. So far as I have been able to
make out, all communication between the yolk-sack and the alimentary
tract is completely obliterated very early. In the Trout the
communication between the two is shut off before hatching, and in the
just-hatched Salmon I can find no trace of any vitelline pedicle. The
absorption of the yolk would seem therefore to be effected entirely by
blood-vessels.

The yolk-sack persists long after hatching, and is gradually absorbed.
There is during the stages either just before hatching or shortly
subsequent to hatching (Cyprinus) a rich vascular development in the
mesoblast of the yolk-sack. The blood is at first contained in lacunar
spaces, but subsequently it becomes confined to definite channels. As
to its exact relations to the vascular system of the embryo more
observations seem to be required.

The following account is given by Rathke (No. 72*) and Lereboullet
(No. 71). At first a subintestinal vein (_vide_ chapter on
Circulation) falls into the lacunæ of the yolk-sack, and the blood
from these is brought back direct to the heart. At a later period,
when the liver is developed, the subintestinal vessel breaks up into
capillaries in the liver, thence passes into the yolk-sack, and from
this to the heart. An artery arising from the aorta penetrates the
liver, and there breaks up into capillaries continuous with those of
the yolk-sack. This vessel is perhaps the equivalent of the artery
which supplies the yolk-sack in Elasmobranchii, but it seems possible
that there is some error in the above description.

BIBLIOGRAPHY.

(55) Al. Agassiz. "On the young Stages of some Osseous Fishes. I.
Development of the Tail." _Proceedings of the American Academy of Arts
and Sciences_, Vol. XIII. Presented Oct. 11, 1877.

(56) Al. Agassiz. "II. Development of the Flounders." _Proceedings of
the American Acad. of Arts and Sciences_, Vol. XIV. Presented June,
1878.

(57) K. E. v. Baer. _Untersuchungen über die Entwicklungsgeschichte
der Fische._ Leipzig, 1835.

(58) Ch. van Bambeke. "Premiers effets de la fécondation sur les oeufs
de Poissons: sur l'origine et la signification du feuillet muqueux ou
glandulaire chez les Poissons Osseux." _Comptes Rendus des Séances de
l'Académie des Sciences_, Tome LXXIV. 1872.

(59) Ch. van Bambeke. "Recherches sur l'Embryologie des Poissons
Osseux." _Mém. couronnés et Mém. de savants étrangers de l'Académie
roy. Belgique_, Vol. XL. 1875.

(60) E. v. Beneden. "A contribution to the history of the Embryonic
development of the Teleosteans." _Quart. J. of Micr. Sci._, Vol.
XVIII. 1878.

(61) E. Calberla. "Zur Entwicklung des Medullarrohres u. d. Chorda
dorsalis d. Teleostier u. d. Petromyzonten." _Morphologisches
Jahrbuch_, Vol. III. 1877.

(62) A. Götte. "Beiträge zur Entwicklungsgeschichte der Wirbelthiere."
_Archiv f. mikr. Anat._, Vol. IX. 1873.

(63) A. Götte. "Ueber d. Entwicklung d. Central-Nervensystems der
Teleostier." _Archiv f. mikr. Anat._, Vol. XV. 1878.

(64) A. Götte. "Entwick. d. Teleostierkeime." _Zoologischer Anzeiger_,
No. 3. 1878.

(65) W. His. "Untersuchungen über die Entwicklung von Knochenfischen,
etc." _Zeit. f. Anat. u. Entwicklungsgeschichte_, Vol. I. 1876.

(66) W. His. "Untersuchungen über die Bildung des Knochenfischembryo
(Salmen)." _Archiv f. Anat. u. Physiol._, 1878.

(67) E. Klein. "Observations on the early Development of the Common
Trout." _Quart. J. of Micr. Science_, Vol. XVI. 1876.

(68) C. Kupffer. "Beobachtungen über die Entwicklung der
Knochenfische." _Archiv f. mikr. Anat._, Bd. IV. 1868.

(69) C. Kupffer. _Ueber Laichen u. Entwicklung des Ostsee-Herings._
Berlin, 1878.

(70) M. Lereboullet. "Recherches sur le développement du brochet de la
perche et de l'écrevisse." _Annales des Sciences Nat._, Vol. I.,
Series IV. 1854.

(71) M. Lereboullet. "Recherches d'Embryologie comparée sur le
développement de la Truite." _An. Sci. Nat._, quatrième série, Vol.
XVI. 1861.

(72) T. Oellacher. "Beiträge zur Entwicklungsgeschichte der
Knochenfische nach Beobachtungen am Bachforellenei." _Zeit. f. wiss.
Zool._, Vol. XXII., 1872, and Vol. XXIII., 1873.

(72*) H. Rathke. _Abh. z. Bildung u. Entwick. d. Menschen u. Thiere._
Leipzig, 1832-3. Part II. Blennius.

(73) Reineck. "Ueber die Schichtung des Forellenkeims." _Archiv f.
mikr. Anat._, Bd. V. 1869.

(74) S. Stricker. "Untersuchungen über die Entwicklung der
Bachforelle." _Sitzungsberichte der Wiener k. Akad. d. Wiss._, 1865.
Vol. LI. Abth. 2.

(75) Carl Vogt. "Embryologie des Salmones." _Histoire Naturelle des
Poissons de l'Europe Centrale._ L. Agassiz. 1842.

(76) C. Weil. "Beiträge zur Kenntniss der Knochenfische."
_Sitzungsber. der Wiener kais. Akad. der Wiss._, Bd. LXVI. 1872.

CHAPTER V.

CYCLOSTOMATA[23].

[23] The following classification of the Cyclostomata is employed
in the present chapter:

I. Hyperoartia ex. Petromyzon.
II. Hyperotreta ex. Myxine, Bdellostoma.

Petromyzon is the only type of this degenerated but primitive group of
Fishes the development of which has been as yet studied[24].

[24] The present chapter is in the main founded upon observations
which I was able to make in the spring of 1880 upon the
development of Petromyzon Planeri. Mr Scott very kindly looked
over my proof-sheets and made a number of valuable suggestions,
and also sent me an early copy of his preliminary note (No. 87),
which I have been able to make use of in correcting my
proof-sheets.

The development does not however throw any light on the relationships
of the group. The similarity of the mouth and other parts of
Petromyzon to those of the Tadpole probably indicates that there
existed a common ancestral form for the Cyclostomata and Amphibia.
Embryology does not however add anything to the anatomical evidence on
this subject. The fact of the segmentation being complete was at one
time supposed to indicate an affinity between the two groups; but the
discovery that the segmentation is also complete in the Ganoids
deprives this feature in the development of any special weight. In the
formation of the layers and in most other developmental characters
there is nothing to imply a special relationship with the Amphibia,
and in the mode of formation of the nervous system Petromyzon exhibits
a peculiar modification, otherwise only known to occur in Teleostei
and Lepidosteus.

Dohrn[25] was the first to bring into prominence the degenerate
character of the Cyclostomata. I cannot however assent to his view
that they are descended from a relatively highly-organized type of
Fish. It appears to me almost certain that they belong to a group of
fishes in which a true skeleton of branchial bars had not become
developed, the branchial skeleton they possess being simply an
extra-branchial system; while I see no reason to suppose that a true
branchial skeleton has disappeared. If the primitive Cyclostomata had
not true branchial bars, they could not have had jaws, because jaws
are essentially developed from the mandibular branchial bar. These
considerations, which are supported by numerous other features of
their anatomy, such as the character of the axial skeleton, the
straightness of the intestinal tube, the presence of a subintestinal
vein etc., all tend to prove that these fishes are remnants of a
primitive and prægnathostomatous group. The few surviving members of
the group have probably owed their preservation to their parasitic or
semiparasitic habits, while the group as a whole probably disappeared
on the appearance of gnathostomatous Vertebrata.

[25] _Der Ursprung d. Wirbelthiere_, etc. Leipzig, 1875.

The ripe ovum of Petromyzon Planeri is a slightly oval body of about 1
mm. in diameter. It is mainly formed of an opaque nearly white yolk,
invested by a membrane composed of an inner perforated layer, and an
outer structureless layer. There appears to be a pore perforating the
inner layer at the formative pole, which may be called a micropyle
(Kupffer and Benecke, No. 79). Enclosing the egg-membranes there is
present a mucous envelope, which causes the egg, when laid, to adhere
to stones or other objects.

[FIG. 37. LONGITUDINAL VERTICAL SECTION THROUGH AN EMBRYO OF
PETROMYZON PLANERI OF 136 HOURS.

_me._ mesoblast; _yk._ yolk-cells; _al._ alimentary tract; _bl._
blastopore; _s.c._ segmentation cavity.]

Impregnation is effected by the male attaching itself by its suctorial
mouth to the female. The attached couple then shake together; and, as
they do so, they respectively emit from their abdominal pores ova and
spermatozoa which pass into a hole previously made[26].

[26] Artificial impregnation may be effected without difficulty
by squeezing out into the same vessel the ova and spermatozoa of
a ripe female and male. The fertilized eggs are easily reared.
Petromyzon Planeri breeds during the second half of April.

The segmentation is total and unequal, and closely resembles that in
the Frog's egg (Vol. II. p. 96). The upper pole is very slightly
whiter than the lower. A segmentation cavity is formed very early, and
is placed between the small cells of the upper pole and the large
cells of the lower pole. It is proportionately larger than in the
Frog; and the roof eventually thins out so as to be formed of a single
row of small cells. At the sides of the segmentation cavity there are
always several rows of small cells, which gradually merge into the
larger cells of the lower pole of the egg. The segmentation is
completed in about fifty hours.

[FIG. 38. TRANSVERSE SECTION THROUGH A PETROMYZON EMBRYO 160 HOURS
AFTER IMPREGNATION.

_ep._ epiblast; _al._ mesenteron; _yk._ yolk-cells; _ms._
mesoblast.]

The segmentation is followed by an asymmetrical invagination (fig. 37)
which leads to a mode of formation of the hypoblast fundamentally
similar to that in the Frog. The process has been in the main
correctly described by M. Schultze (No. 81).

On the border between the large and small cells of the embryo, at a
point slightly below the segmentation cavity, a small circular pit
appears; the roof of which is formed by an infolding of the small
cells, while the floor is formed of the large cells. This pit is the
commencing mesenteron. It soon grows deeper (fig. 37, _al_) and
extends as a well-defined tube (shewn in transverse section in fig.
38, _al_) in the direction of the segmentation cavity. In the course
of the formation of the mese

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