The works of Francis Maitland Balfour, Volume 1 (of 4)
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Title: The works of Francis Maitland Balfour, Volume 1 (of 4)
Separate memoirs
Author: Francis M. Balfour
Editor: Sir M. Foster
Adam Sedgwick
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*** START OF THE PROJECT GUTENBERG EBOOK THE WORKS OF FRANCIS MAITLAND BALFOUR, VOLUME 1 (OF 4) ***
THE WORKS
OF
FRANCIS MAITLAND BALFOUR.
VOL. I.
Memorial Edition.
Cambridge:
PRINTED BY C. J. CLAY, M.A. AND SON,
AT THE UNIVERSITY PRESS.
[Illustration: Sketch of Francis Maitland Balfour]
Memorial Edition.
THE WORKS
OF
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. I.
SEPARATE MEMOIRS.
London:
MACMILLAN AND CO.
1885
[_The Right of Translation is reserved._]
PREFACE.
Upon the death of Francis Maitland Balfour, a desire very naturally arose
among his friends and admirers to provide some memorial of him. And, at a
public meeting held at Cambridge in October 1882, the Vice-Chancellor
presiding, and many distinguished men of science being present, it was
decided to establish a 'Balfour Fund' the proceeds of which should be
applied: firstly to maintain a studentship, the holder of which should
devote himself to original research in Biology, especially in Animal
Morphology, and secondly, 'by occasional grants of money, to further in
other ways original research in the same subject'. The sum of £8446 was
subsequently raised; this was, under certain conditions, entrusted to and
accepted by the University of Cambridge; and the first 'Balfour student'
was appointed in October 1883.
The publication of Balfour's works in a collected form was not proposed as
an object on which part of the fund should be expended, since his family
had expressed their wish to take upon themselves the charge of arranging
for a memorial edition of their brother's scientific writings. That
edition, with no more delay than circumstances have rendered necessary, is
now laid before the public. It comprises four volumes.
The first volume contains, in chronological order, all Balfour's scattered
original papers, including those published by him in conjunction with his
pupils, as well as the Monograph on the Elasmobranch Fishes. The last
memoir in the volume, that on the Anatomy and Development of Peripatus
Capensis, was published after his death, from his notes and drawings, with
additions by Prof. Moseley and Mr Adam Sedgwick, who prepared the
manuscript for publication. To the volume is prefixed an introductory
biographical notice.
The second and third volumes are the two volumes of the Comparative
Embryology reprinted from the original edition without alteration, save the
correction of obvious misprints and omissions.
The fourth volume contains the plates illustrating the memoirs contained in
Vol. 1. We believe that we are consulting the convenience of readers in
adopting this plan, rather than in distributing the plates among the
memoirs to which they belong. To assist the reader the explanations of
these plates have been given twice: at the end of the memoir to which they
belong (in the case of the Monograph on Elasmobranch Fishes at the end of
each separate chapter), and in the volume of plates.
All the figures of these plates had to be redrawn on the stone, and our
best thanks are due to the Cambridge Scientific Instrument Company for the
pains which they have taken in executing this work. We are also indebted to
the Committee of Publication of the Zoological Society for the gift of
electrotypes of the woodcuts illustrating memoir no. XX. of Vol. 1.
Several photographs of Balfour, taken at different times of his life, the
last shortly before his death, are in the possession of his relatives and
friends; but these, in the opinion of many, leave much to be desired.
There is also a portrait of him in oils painted since his death by Mr John
Collier, A.R.A., and Herr Hildebrand of Florence has executed a posthumous
bust in bronze[1]. The portrait which forms the frontispiece of Vol. 1. has
been drawn on stone by Mr E. Wilson of the Cambridge Scientific Instrument
Company, after the latest photograph. Should it fail, in the eyes of those
who knew Balfour well, to have reproduced with complete success his
features and expression, we would venture to ask them to bear in mind the
acknowledged difficulties of posthumous portraiture.
Footnote 1: In possession of the family. Copies also exist in
the Library of Trinity College, and in the Morphological
Laboratory, at Cambridge.
TABLE OF CONTENTS.
PAGE
PREFACE i
INTRODUCTION 1
1872
I. On some points in the Geology of the East Lothian
Coast. By G. W. and F. M. BALFOUR 25
1873
II. The development and growth of the layers of the
blastoderm. With Plate 1 29
III. On the disappearance of the Primitive Groove in the
Embryo Chick. With Plate 1 41
IV. The development of the blood-vessels of the Chick.
With Plate 2 47
1874
V. A preliminary account of the development of the
Elasmobranch Fishes. With Plates 3 and 4 60
1875
VI. A comparison of the early stages in the development
of Vertebrates. With Plate 5 112
VII. On the origin and history of the urinogenital organs
of Vertebrates 135
VIII. On the development of the spinal nerves in Elasmobranch
Fishes. With Plates 22 and 23 168
1876
IX. On the spinal nerves of Amphioxus 197
1876-78
X. A Monograph on the development of Elasmobranch
Fishes. With Plates 6-21 203
1878
XI. On the phenomena accompanying the maturation and
impregnation of the ovum 521
XII. On the structure and development of the vertebrate
ovary. With Plates 24, 25, 26 549
1879
XIII. On the existence of a Head-kidney in the Embryo Chick,
and on certain points in the development of the
Müllerian duct. By F. M. BALFOUR and A. SEDGWICK.
With Plates 27 and 28 618
XIV. On the early development of the Lacertilia, together
with some observations on the nature and relations of
the primitive Streak. With Plate 29 644
XV. On certain points in the Anatomy of Peripatus Capensis 657
XVI. On the morphology and systematic position of the
Spongida 661
1880
XVII. Notes on the development of the Araneina. With Plates
30, 31, 32 668
XVIII. On the spinal nerves of Amphioxus 696
XIX. Address to the Department of Anatomy and Physiology
of the British Association for the Advancement of
Science 698
1881
XX. On the development of the skeleton of the paired fins of
Elasmobranchii, considered in relation to its bearings
on the nature of the limbs of the Vertebrata. With
Plate 33 714
XXI. On the evolution of the Placenta, and on the possibility
of employing the characters of the Placenta in the
classification of the Mammalia 734
1882
XXII. On the structure and development of Lepidosteus. By
F. M. BALFOUR and W. N. PARKER. With Plates
34-42 738
XXIII. On the nature of the organ in Adult Teleosteans and
Ganoids which is usually regarded as the Head-kidney
or Pronephros 848
XXIV. A renewed study of the germinal layers of the Chick. By
F. M. BALFOUR and F. DEIGHTON. With Plates
43, 44, 45 854
POSTHUMOUS, 1883
XXV. The Anatomy and Development of Peripatus Capensis.
Edited by H. N. MOSELEY and A. SEDGWICK. With
Plates 46-53 871
Francis Maitland Balfour, the sixth child and third son of James Maitland
Balfour of Whittinghame, East Lothian, and Lady Blanche, daughter of the
second Marquis of Salisbury, was born at Edinburgh, during a temporary stay
of his parents there, on the 10th November, 1851. He can hardly be said to
have known his father, who died of consumption in 1856, at the early age of
thirty-six, and who spent the greater part of the last two years of his
life at Madeira, separated from the younger children who remained at home.
He fancied at one time that he had inherited his father's constitution; and
this idea seems to have spurred him on to achieve early what he had to do.
But, though there was a period soon after he went to College, during which
he seemed delicate, and the state of his health caused considerable anxiety
to his friends, he eventually became fairly robust, and that in spite of
labours which greatly taxed his strength.
The early years of his life were spent chiefly at Whittinghame under the
loving care of his mother. She made it a point to attempt to cultivate in
all her children some taste for natural science, especially for natural
history, and in this she was greatly helped by the boys' tutor, Mr J. W.
Kitto. They were encouraged to make collections and to form a museum, and
the fossils found in the gravel spread in front of the house served as the
nucleus of a geological series. Frank soon became greatly interested in
these things, and indeed they may be said to have formed the beginnings of
his scientific career. At all events there was thus awakened in him a love
for geology, which science continued to be his favorite study all through
his boyhood, and interested him to the last. He was most assiduous in
searching for fossils in the gravel and elsewhere, and so great was his
love for his collections that while as yet quite a little boy the most
delightful birthday present he could think of was a box with trays and
divisions to hold his fossils and specimens. His mother, thinking that his
fondness for fossils was a passing fancy and that he might soon regret the
purchase of the box, purposely delayed the present. But he remained
constant to his wish and in time received his box. He must at this time
have been about seven or eight years old. In the children's museum, which
has been preserved, there are specimens labelled with his childish
round-hand, such as a piece of stone with the label "marks of some shels;"
and his sister Alice, who was at that time his chief companion, remembers
discussing with him one day after the nursery dinner, when he was about
nine years old, whether it were better to be a geologist or a naturalist,
he deciding for the former on the ground that it was better to do one thing
thoroughly than to attempt many branches of science and do them
imperfectly.
Besides fossils, he collected not only butterflies, as do most boys at some
time or other, but also birds; and he with his sister Alice, being
instructed in the art of preparing and preserving skins, succeeded in
making a very considerable collection. He thus acquired before long not
only a very large but a very exact knowledge of British birds.
In the more ordinary work of the school-room he was somewhat backward. This
may have been partly due to the great difficulty he had in learning to
write, for he was not only left-handed but, in his early years, singularly
inapt in acquiring particular muscular movements, learning to dance being a
great trouble to him. Probably however the chief reason was that he failed
to find any interest in the ordinary school studies. He fancied that the
family thought him stupid, but this does not appear to have been the case.
In character he was at this time quick tempered, sometimes even violent,
and the energy which he shewed in after life even thus early manifested
itself as perseverance, which, when he was crossed, often took on the form
of obstinacy, causing at times no little trouble to his nurses and tutors.
But he was at the same time warm-hearted and affectionate; full of strong
impulses, he disliked heartily and loved much, and in his affections was
wonderfully unselfish, wholly forgetting himself in his thought for others,
and ready to do things which he disliked to please those whom he loved.
Though, as we have said, somewhat clumsy, he was nevertheless active and
courageous; in learning to ride he shewed no signs of fear, and boldly put
his pony to every jump which was practicable.
In 1861 he was sent to the Rev. C. G. Chittenden's preparatory school at
Hoddesden in Hertfordshire, and here the qualities which had been already
visible at home became still more obvious. He found difficulty not only in
writing but also in spelling, and in the ordinary school-work he took but
little interest and made but little progress.
In 1865 he was moved to Harrow and placed in the house of the Rev. F.
Rendall. Here, as at Hoddesden, he did not show any great ability in the
ordinary school studies, though as he grew older his progress became more
marked. But happily he found at Harrow an opportunity for cultivating that
love of scientific studies which was yearly growing stronger in him. Under
the care of one of the Masters, Mr G. Griffith, the boys at Harrow were
even then taught the elements of natural science. The lessons were at that
time, so to speak, extra-academical, carried on out of school hours;
nevertheless, many of the boys worked at them with diligence and even
enthusiasm, and among these Balfour became conspicuous, not only by his
zeal but by his ability. Griffith was soon able to recognize the power of
his new pupil, and thus early began to see that the pale, earnest, somewhat
clumsy-handed lad, though he gave no promise of being a scholar in the
narrower sense of the word, had in him the makings of a man of science.
Griffith chiefly confined his teaching to elementary physics and chemistry
with some little geology, but he also encouraged natural history studies
and began the formation of a museum of comparative anatomy. Balfour soon
began to be very zealous in dissecting animals, and was especially
delighted when the Rev. A. C. Eaton, the well-known entomologist, on a
visit to Harrow, initiated Griffith's pupils in the art of dissecting under
water. The dissection of a caterpillar in this way was probably an epoch in
Balfour's life. Up to that time his rough examination of such bodies had
revealed to him nothing more than what in school-boy language he spoke of
as "squash;" but when under Eaton's deft hands the intricate organs of the
larval Arthropod floated out under water and displayed themselves as a
labyrinth of threads and sheets of silvery whiteness a new world of
observation opened itself up to Balfour, and we may probably date from this
the beginning of his exact morphological knowledge.
While thus learning the art of observing, he was at the same time
developing his power of thinking. He was by nature fond of argument, and
defended with earnestness any opinions which he had been led to adopt. He
was very active in the Harrow Scientific Society, reading papers, taking
part in the discussions, and exhibiting specimens. He gained in 1867 a
prize for an essay on coal, and when, in 1868, Mr Leaf offered a prize (a
microscope) "for the best account of some locality visited by the writer
during the Easter Holidays," two essays sent in, one by Balfour, the other
by his close friend, Mr Arthur Evans, since well known for his researches
in Illyria, were found to be of such unusual merit that Prof. Huxley was
specially requested to adjudicate between them. He judged them to be of
equal merit, and a prize was given to each. The subject of Balfour's essay
was "The Geology and Natural History of East Lothian." When biological
subjects were discussed at the Scientific Society, Balfour appears to have
spoken as a most uncompromising opponent of the views of Mr Charles Darwin,
little thinking that in after life his chief work would be to develop and
illustrate the doctrine of evolution.
The years at Harrow passed quickly away, Balfour making fair, but perhaps
not more than fair, progress in the ordinary school learning. In due course
however he reached the upper sixth form, and in his last year, became a
monitor. At the same time his exact scientific knowledge was rapidly
increasing. Geology still continued to be his favorite study, and in this
he made no mean progress. During his last years at Harrow he and his
brother Gerald worked out together some views concerning the geology of
their native county. These views they ultimately embodied in a paper, which
was published in their joint names in the _Geological Magazine_ for 1872,
under the title of "Some Points in the Geology of the East Lothian Coast,"
and which was in itself a work of considerable promise. Geology however was
beginning to find a rival in natural history. Much of his holiday time was
now spent in dredging for marine animals along the coast off Dunbar. Each
specimen thus obtained was carefully determined and exact records were kept
of the various 'finds,' so that the dredgings (which were zealously
continued after he had left Harrow and gone to Cambridge) really
constituted a serious study of the fauna of this part of the coast. They
also enabled him to make a not inconsiderable collection of shells, in the
arrangement of which he was assisted by his sister Evelyn, of crustacea and
of other animals.
Both to the masters and to his schoolfellows he became known as a boy of
great force of character. Among the latter his scrupulous and unwavering
conscientiousness made him less popular perhaps than might have been
expected from his bright kindly manner and his unselfish warmheartedness.
In the incidents of school life a too strict conscience is often an
inconvenience, and the sternness and energy with which Balfour denounced
acts of meanness and falsehood were thought by some to be unnecessarily
great. He thus came to be feared rather than liked by many, and
comparatively few grew to be sufficiently intimate with him to appreciate
the warmth of his affections and the charm of his playful moments.
At the Easter of 1870 he passed the entrance examination at Trinity
College, Cambridge, and entered into residence in the following October.
His college tutor was Mr J. Prior, but he was from the first assisted and
guided in his studies by his friend, Mr Marlborough Pryor, an old Harrow
boy, who in the same October had been, on account of his distinction in
Natural Science, elected a Fellow of the College, in accordance with
certain new regulations which then came into action for the first time, and
which provided that every three years one of the College Fellowships should
be awarded for excellence in some branch or branches of Natural Science, as
distinguished from mathematics, pure or mixed. During the whole of that
year and part of the next Mr Marlborough Pryor remained in residence, and
his influence in wisely directing Balfour's studies had a most beneficial
effect on the latter's progress.
During his first term Balfour was occupied in preparation for the Previous
Examination; and this he successfully passed at Christmas. After that he
devoted himself entirely to Natural Science, attending lectures on several
branches. During the Lent term he was a very diligent hearer of the
lectures on Physiology which I was then giving as Trinity Prælector, having
been appointed to that post in the same October that Balfour came into
residence. At this time he was not very strong, and I remember very well
noticing among my scanty audience, a pale retiring student, whose mind
seemed at times divided between a desire to hear the lecture and a feeling
that his frequent coughing was growing an annoyance to myself and the
class. This delicate-looking student, I soon learnt, was named Balfour, and
when the Rev. Coutts Trotter, Mr Pryor and myself came to examine the
candidates for the Natural Science Scholarships which were awarded at
Easter, we had no difficulty in giving the first place to him. In point of
knowledge, and especially in the thoughtfulness and exactitude displayed in
his papers and work, he was very clearly ahead of his competitors.
During the succeeding Easter term and the following winter he appears to
have studied physics, chemistry, geology and comparative anatomy, both
under Mr Marlborough Pryor and by means of lectures. He also continued to
attend my lectures, but though I gradually got to know him more and more we
did not become intimate until the Lent term of 1872. He had been very much
interested in some lectures on embryology which I had given, and, since
Marlborough Pryor had left or was about to leave Cambridge, he soon began
to consult me a good deal about his studies. He commenced practical
histological and embryological work under me, and I remember very vividly
that one day when we were making a little excursion in search of nests and
eggs of the stickleback in order that he might study the embryology of
fishes, he definitely asked my opinion as to whether he might take up a
scientific career with a fair chance of success. I had by this time formed
a very high opinion of his abilities, and learning then for the first time
that he had an income independent of his own exertions, my answer was very
decidedly a positive one. Soon after, feeling more and more impressed with
his power and increasingly satisfied both with his progress in biological
studies and his sound general knowledge of other sciences, anxious also, it
may be, at the same time that as much original inquiry as possible should
be carried on at Cambridge in my department, I either suggested to him or
acquiesced in his own suggestion that he should at once set to work on some
distinct research; and as far as I remember the task which I first proposed
to him was an investigation of the layers of the blastoderm in the chick.
It must have been about the same time that I proposed to him to join me in
preparing for publication a small work on Embryology, the materials for
this I had ready to hand in a rough form as lectures which I had previously
given. To this proposal he enthusiastically assented, and while the lighter
task of writing what was to be written fell to me, he undertook to work
over as far as was possible the many undetermined points and unsatisfactory
statements across which we were continually coming.
During his two years at College his health had improved; though still
hardly robust and always in danger of overworking himself, he obviously
grew stronger. He rejoiced exceedingly in his work, never tiring of it, and
was also making his worth felt among his fellow students, and especially
perhaps among those of his own college whose studies did not lie in the
same direction as his own. At this time he must have been altogether happy,
but a sorrow now came upon him. His mother, to whom he was passionately
attached, and to whose judicious care in his early days not only the right
development of his strong character but even his scientific leanings were
due, had for some time past been failing in health, though her condition
caused no immediate alarm. In May 1872, however, she died quite suddenly
from unsuspected heart disease. Her loss was a great blow to him, and for
some time afterward I feared his health would give way; but he bore his
grief quietly and manfully and threw himself with even increased vigour
into his work.
During the academic session of 1872-3, he continued steadily at work at his
investigations, and soon began to make rapid progress. At the beginning he
had complained to me about what he considered his natural clumsiness, and
expressed a fear that he should never be able to make satisfactory
microscopic sections; as to his being able to make drawings of his
dissections and microscopical preparations, he looked upon that at first as
wholly impossible. I need hardly say that in time he acquired great skill
in the details of microscopical _technique_, and that his drawings, if
wanting in so-called artistic finish, were always singularly true and
instructive. While thus struggling with the details which I could teach
him, he soon began to manifest qualities which no teacher could give him. I
remember calling his attention to Dursy's paper on the Primitive Streak,
and suggesting that he should work the matter over, since if such a
structure really existed, it must, most probably, have great morphological
significance. I am free to confess that I myself rather doubted the matter,
and a weaker student might have been influenced by my preconceptions.
Balfour, however, thus early had the power of seeing what existed and of
refusing to see what did not exist. He was soon able to convince me that
Dursy's streak was a reality, and the complete working out of its
significance occupied his thoughts to the end of his days.
The results of these early studies were made known in three papers which
appeared in the _Quarterly Journal of Microscopical Science_ for July 1873,
and will be found in the beginning of this volume. The summer and autumn of
that year were spent partly in a visit to Finland, in company with his
friend and old school-fellow Mr Arthur Evans, and partly in formal
preparation for the approaching Tripos examination. Into this preparation
Balfour threw himself with characteristic energy, and fully justified my
having encouraged his spending so much of the preceding time in original
research, not only by the rapidity with which he accumulated the stock of
knowledge of various kinds necessary for the examination but also by the
manner in which he acquitted himself at the trial itself. At that time the
position of the candidates in the Natural Sciences Tripos was determined by
the total number of marks, and Balfour was placed second, the first place
being gained by H. Newell Martin of Christ's College, now Professor at
Baltimore, U.S.A. In the examination, in which I took part, Balfour did not
write much, and he had not yet learnt the art of putting his statements in
the best possible form; he won his position chiefly by the firm thought and
clear insight which was present in almost all his answers.
The examination was over in the early days of Dec. 1873 and Balfour was now
free to devote himself wholly to his original work. Happily, the University
had not long before secured the use of two of the tables at the then
recently founded Stazione Zoologica at Naples. And upon the nomination of
the University, Balfour, about Christmas, started for Naples in company
with his friend Mr A. G. Dew-Smith, also of Trinity College. The latter was
about to carry on some physiological observations; Balfour had set himself
to work out as completely as he could the embryology of Elasmobranch
fishes, about which little was at that time known, but which, from the
striking characters of the adult animals could not help proving of interest
and importance.
From his arrival there at Christmas 1873 until he left in June 1874, he
worked assiduously, and with such success, that as the result of the
half-year's work he had made a whole series of observations of the greatest
importance. Of these perhaps the most striking were those on the
development of the urogenital organs, on the neurenteric canal, on the
development of the spinal nerves, on the formation of the layers and on the
phenomena of segmentation, including a history of the behaviour of nuclei
in cell division. He returned home laden with facts and views both novel
and destined to influence largely the progress of embryology.
In August of the same year he attended the meeting of the British
Association for the Advancement of Science at Belfast; and the account he
then gave of his researches formed one of the most important incidents at
the Biological Section on that occasion.
In the September of that year the triennial fellowship for Natural Science
was to be awarded at Trinity College, and Balfour naturally was a
candidate. The election was, according to the regulations, to be determined
partly by the result of an examination in various branches of science, and
partly by such evidence of ability and promise as might be afforded by
original work, published or in manuscript. He spent the remainder of the
autumn in preparation for this examination. But when the examination was
concluded it was found that in his written answers he had not been very
successful; he had not even acquitted himself so well as in the Tripos of
the year before, and had the election been determined by the results of the
examination alone, the examiners would have been led to choose the
gentleman who was Balfour's only competitor. The original work however
which Balfour sent in, including a preliminary account of the discoveries
made at Naples, was obviously of so high a merit and was spoken of in such
enthusiastic terms by the External Referee Prof. Huxley, that the examiners
did not hesitate for a moment to neglect altogether the formal written
answers (and indeed the papers of questions were only introduced as a
safeguard, or as a resource in case evidence of original power should be
wanted) and unanimously recommended him for election. Accordingly he was
elected Fellow in the early days of October.
Almost immediately after, the little book on Embryology appeared, on which
he and I had been at work, he doing his share even while his hands and mind
were full of the Elasmobranch inquiry. The title-page was kept back some
little time in order that his name might appear on it with the addition of
Fellow of Trinity, a title of which he was then, and indeed always
continued to be, proud. He also published in the October number of the
_Quarterly Journal of Microscopical Science_ a preliminary account of his
Elasmobranch researches.
He and his friends thought that after these almost incessant labours, and
the excitement necessarily contingent upon the fellowship election, he
needed rest and change. Accordingly on the 17th of October he started with
his friend Marlborough Pryor on a voyage to the west coast of South
America. They travelled thither by the Isthmus of Panama, visited Peru and
Chili, and returned home along the usual route by the Horn; reaching
England some time in Feb. 1875.
Refreshed by this holiday, he now felt anxious to complete as far as
possible his Elasmobranch work, and very soon after his return home, in
fact in March, made his way again to Naples, where he remained till the hot
weather set in in May. On his return to Cambridge, he still continued
working on the Elasmobranchii, receiving material partly from Naples,
partly from the Brighton Aquarium, the then director of which, Mr Henry
Lee, spared no pains to provide him both with embryo and adult fishes.
While at Naples, he communicated to the Philosophical Society at Cambridge
a remarkable paper on "The Early Stages of Vertebrates," which was
published in full in the _Quarterly Journal of Microscopical Science_,
July, 1875; he also sent me a paper on "The Development of the Spinal
Nerves", which I communicated to the Royal Society, and which was
subsequently published in the _Philosophical Transactions_ of 1876. He
further wrote in the course of the summer and published in the _Journal of
Anatomy and Physiology_ in October, 1875, a detailed account of his
"Observations and Views on the Development of the Urogenital Organs."
Some time in August of the same year he started in company with Mr Arthur
Evans and Mr J. F. Bullar for a second trip to Finland, the travellers on
this occasion making their way into regions very seldom visited, and having
to subsist largely on the preserved provisions which they carried with
them, and on the produce of their rods and guns. From a rough diary which
Balfour kept during this trip it would appear that while enjoying heartily
the fun of the rough travelling, he occupied himself continually with
observations on the geology and physical phenomena of the country, as well
as on the manners, antiquities, and even language of the people. It was one
of his characteristic traits, a mark of the truly scientific bent of his
mind, of his having, as Dohrn soon after Balfour's first arrival at Naples
said, 'a real scientific head,' that every thing around him wherever he
was, incited him to careful exact observation, and stimulated him to
thought.
In the early part of the Long Vacation of the same year he had made his
first essay in lecturing, having given a short course on Embryology in a
room at the New Museums, which I then occupied as a laboratory. Though he
afterwards learnt to lecture with great clearness he was not by nature a
fluent speaker, and on this occasion he was exceedingly nervous. But those
who listened to him soon forgot these small defects as they began to
perceive the knowledge and power which lay in their new teacher.
Encouraged by the result of this experiment, he threw himself, in spite of
the heavy work which the Elasmobranch investigation was entailing, with
great zeal into an arrangement which Prof. Newton, Mr J. W. Clark and
myself had in course of the summer brought about, that he and Mr A. Milnes
Marshall, since Professor at Owens College, Manchester, should between them
give a course on Animal Morphology, with practical instruction, Prof.
Newton giving up a room in the New Museums for the purpose.
In the following October (1875) upon Balfour's return from Finland, these
lectures were accordingly begun and carried on by the two lecturers during
the Michaelmas and Lent Terms. The number of students attending this first
course, conducted on a novel plan, was, as might be expected, small, but
the Lent Term did not come to an end before an enthusiasm for morphological
studies had been kindled in the members of the class.
The ensuing Easter term (1876) was spent by Balfour at Naples, in order
that he might carry on towards completion his Elasmobranch work. He had by
this time determined to write as complete a monograph as he could of the
development of these fishes, proposing to publish it in instalments in the
_Journal of Anatomy and Physiology_, and subsequently to gather together
the several papers into one volume. The first of these papers, dealing with
the ovum, appeared in Jan. 1876; most of the numbers of the Journal during
that and the succeeding year contained further portions; but the complete
monograph did not leave the publisher's hands until 1878.
He returned to England with his pupil and friend Mr J. F. Bullar some time
in the summer; on their way home they passed through Switzerland, and it
was during the few days which he then spent in sight of the snow-clad hills
that the beginnings of a desire for that Alpine climbing, which was
destined to be so disastrous, seem to have been kindled in him.
In October, 1876, he resumed the lectures on Morphology, taking the whole
course himself, his colleague, Mr Marshall, having meanwhile left
Cambridge. Indeed, from this time onward, he may be said to have made these
lectures, in a certain sense, the chief business of his life. He lectured
all three terms, devoting the Michaelmas and Lent terms to a systematic
course of Animal Morphology, and the Easter term to a more elementary
course of Embryology. These lectures were given under the auspices of Prof.
Newton; but Balfour's position was before long confirmed by his being made
a Lecturer of Trinity College, the lectures which he gave at the New
Museums, and which were open to all students of the University, being
accepted in a liberal spirit by the College as equivalent to College
Lectures. He very soon found it desirable to divide the morphological
course into an elementary and an advanced course, and to increase the
number of his lectures from three to four a week. Each lecture was followed
by practical work, the students dissecting and examining microscopically,
an animal or some animals chosen as types to illustrate the subject-matter
of the lecture; and although Balfour had the assistance at first of one[2],
and ultimately of several demonstrators, he himself put his hand to the
plough, and after the lecture always spent some time in the laboratory
among his pupils. Had Balfour been only an ordinary man, the zeal and
energy which he threw into his lectures, and into the supervision of the
practical work, added to the almost brotherly interest which he took in the
individual development of every one of the pupils who shewed any love
whatever for the subject, would have made him a most successful teacher.
But his talents and powers were such as could not be hid even from
beginners. His extensive and exact knowledge, the clearness with which in
spite of, or shall I not rather say, by help of a certain want of fluency,
he explained difficult and abstruse matters, the trenchant way in which he
lay bare specious fallacies, and the presence in almost his every word of
that power which belongs only to the man who has thought out for himself
everything which he says, these things aroused and indeed could hardly fail
to arouse in his hearers feelings which, except in the case of the very
dullest, grew to be those of enthusiasm. His class, at first slowly, but
afterwards more rapidly, increased in numbers, and, what is of more
importance, grew in quality. The room allotted to him soon became far too
small and steps were taken to provide for him, for myself, whose wants were
also urgent, and for the biological studies generally, adequate
accommodation; but it was not until Oct. 1877 that we were able to take
possession of the new quarters.
Footnote 2: His first Demonstrator up to Christmas 1877, was
Mr J. F. Bullar. In Jan. 1878, Mr Adam Sedgwick took the post
of Senior Demonstrator, and held it until Balfour's death.
Even this new accommodation soon became insufficient, and in the spring of
1882 a new morphological laboratory was commenced in accordance with plans
suggested by himself. He was to have occupied them in the October term,
1883, but did not live to see them finished.
As might have been expected from his own career, he regarded the mere
teaching of what is known as a very small part of his duties as Lecturer;
and as soon as any of his pupils became sufficiently advanced, he urged or
rather led them to undertake original investigations; and he had the
satisfaction before his death of seeing the researches of his pupils (such
as those by Messrs. Bullar, Sedgwick, Mitzikuri, Haddon, Scott, Osborne,
Caldwell, Heape, Weldon, Parker, Deighton and others) carried to a
successful end. In each of these inquiries he himself took part, sometimes
a large part, generally suggesting the problem to be solved, indicating the
methods, and keeping a close watch over the whole progress of the study.
Hence in many cases the published account bears his name as well as that of
the pupil.
In the year 1878 his _Monograph on Elasmobranch Fishes_ was published as a
complete volume, and in the same year he received the honour of being
elected a Fellow of the Royal Society, a distinction which now-a-days does
not often fall to one so young. No sooner was the Monograph completed than
in spite of the labours which his lectures entailed, he set himself to the
great task of writing a complete treatise on _Comparative Embryology_. This
not only laid upon him the heavy burden of gathering together the
observations of others, enormous in number and continually increasing,
scattered through many journals and books, and recorded in many different
languages, as well as of putting them in orderly array, and of winnowing
out the grain from the chaff (though his critical spirit found some relief
in the latter task), but also caused him much labour, inasmuch as at almost
every turn new problems suggested themselves, and demanded inquiry before
he could bring his mind to writing about them. This desire to see his way
straight before him, pursued him from page to page, and while it has
resulted in giving the book an almost priceless value, made the writing of
it a work of vast labour. Many of the ideas thus originated served as the
bases of inquiries worked out by himself or his pupils, and published in
the form of separate papers, but still more perhaps never appeared either
in the book or elsewhere and were carried with him undeveloped and
unrecorded to the grave.
The preparation of this work occupied the best part of his time for the
next three years, the first volume appearing in 1880, the second in 1881.
In the autumn of 1880, he attended the Meeting at Swansea of the British
Association for the Advancement of Science, having been appointed
Vice-President of the Biological Section with charge of the Department of
Anatomy and Physiology. At the Meetings of the Association, especially of
late years, much, perhaps too much, is expected in the direction of
explaining the new results of science in a manner interesting to the
unlearned. Popular expositions were never very congenial to Balfour, his
mind was too much occupied with the anxiety of problems yet to be solved;
he was therefore not wholly at his ease, in his position on this occasion.
Yet his introductory address, though not of a nature to interest a large
mixed audience, was a luminous, brief exposition of the modern development
and aims of embryological investigation.
During these years of travail with the _Comparative Embryology_ the amount
of work which he got through was a marvel to his friends, for besides his
lectures, and the researches, and the writing of the book, new labours were
demanded of him by the University for which he was already doing so much.
Men at Cambridge, and indeed elsewhere as well, soon began to find out that
the same clear insight which was solving biological problems could be used
to settle knotty questions of policy and business. Moreover he united in a
remarkable manner, the power of boldly and firmly asserting and maintaining
his own views, with a frank courteousness which went far to disarm
opponents. Accordingly he found himself before long a member of various
Syndicates, and indeed a very great deal of his time was thus occupied,
especially with the Museums and Library Syndicates, in both of which he
took the liveliest interest. Besides these University duties his time and
energy were also at the service of his College. In the preparation of the
New Statutes, with which about this time the College was much occupied, the
Junior Fellows of the College took a conspicuous share, and among these
Junior Fellows Balfour was perhaps the most active; indeed he was their
leader, and he threw himself into the investigation of the bearings and
probable results of this and that proposed new statute with as much zeal as
if he were attacking some morphological problem.
While he was in the midst of these various labours, his friends often
feared for his strength, for though gradually improving in health after his
first year at Cambridge, he was not robust, and from time to time he seemed
on the point of breaking down. Still, hard as he was working, he was in
reality wisely careful of himself, and as he grew older, paid more and more
attention to his health, daily taking exercise in the form either of
bicycle rides or of lawn-tennis. Moreover he continued to spend some part
of his vacations in travel. Combining business with pleasure, he made
frequent visits to Germany and France, and especially to Naples. The
Christmas of 1876-7 he spent in Greece, that of 1878-9 at Ragusa, where his
old school-fellow and friend Mr Arthur Evans was at that time residing, and
the appointment of his friend Kleinenberg to a Professorship at Messina led
to a journey there. Early in the long vacation of 1880, he went with his
sister, Mrs H. Sidgwick, and her husband to Switzerland, and was joined
there for a short time by his friend and pupil Adam Sedgwick. During this
visit he took his first lessons in Alpine climbing, making several
excursions, some of them difficult and dangerous; and the love of
mountaineering laid so firm a hold upon him, that he returned to
Switzerland later on in the autumn of the same year, in company with his
brother Gerald, and spent some weeks near Zermatt in systematic climbing,
ascending, among other mountains, the Matterhorn and the Weisshorn. In the
following summer, 1881, he and his brother Gerald again visited the Alps,
dividing their time between the Chamonix district and the Bernese Oberland.
On this occasion some of the excursions which they made were of extreme
difficulty, and such as needed not only great presence of mind and bodily
endurance, but also skilful and ready use of the limbs. As a climber indeed
Balfour soon shewed himself fearless, indefatigable, and expert in all
necessary movements as well as full of resources and expedients in the face
of difficulties, so much so that he almost at once took rank among the
foremost of distinguished mountaineers. In spite of his apparent clumsiness
in some matters, he had even as a lad proved himself to be a bold and
surefooted climber. Moreover he had been perhaps in a measure prepared for
the difficulties of Alpine climbing by his experience in deer-stalking.
This sport he had keenly and successfully pursued for many years at his
brother's place in Rosshire. When however about the year 1877, the question
of physiological experiments on animals became largely discussed in public,
he felt that to continue the pursuit of this or any other sport involving,
for the sake of mere pleasure, the pain and death of animals, was
inconsistent with the position which he had warmly taken up, as an advocate
of the right to experiment on animals; and he accordingly from that time
onward wholly gave it up.
His fame as an investigator and teacher, and as a man of brilliant and
powerful parts, was now being widely spread. Pupils came to him, not only
from various parts of England, but from America, Australia and Japan. At
the York Meeting of the British Association for the Advancement of Science,
in August, 1881, he was chosen as one of the General Secretaries. In April,
1881, the honorary degree of LL.D. was conferred upon him by the University
of Glasgow, and in November of the same year the Royal Society gave him one
of the Royal Medals in recognition of his embryological discoveries, and at
the same time placed him on its Council.
At Cambridge he was chosen, in the autumn of 1880, President of the
Philosophical Society, and in the December of that year a brilliant company
were gathered together at the Annual Dinner to do honour to their new young
President. Otherwise nothing as yet had been done for him in his own
University in the way of recognition of his abilities and services; and he
still remained a Lecturer of Trinity College, giving lectures in a
University building. An effort had been made by some of his friends to urge
the University to take some step in this direction; but it was thought at
that time impossible to do anything. In 1881 a great loss fell upon the
sister University of Oxford in the death of Prof. George Rolleston; and
soon after very vigorous efforts were made to induce Balfour to become a
candidate for the vacant chair. The prospect was in many ways a tempting
one, and Balfour seeing no very clear way in the future for him at his own
University, was at times inclined to offer himself, but eventually he
decided to remain at Cambridge. Hardly had this temptation if we may so
call it been overcome when a still greater one presented itself. Through
the lamented death of Sir Wyville Thomson in the winter of 1881-2, the
chair of Natural History at Edinburgh, perhaps the richest and most
conspicuous biological chair in the United Kingdom, became vacant. The post
was in many ways one which Balfour would have liked to hold. The teaching
duties were it is true laborious, but they had in the past been compressed
into a short time, occupying only the summer session and leaving the rest
of the year free, and it seemed probable that this arrangement might be
continued with him. The large emolument would also have been grateful to
him inasmuch as he would have felt able to devote the whole of it to
scientific ends; and the nearness to Whittinghame, his native place and
brother's home, added to the attractions; but what tempted him most was the
position which it would have given him, and the opportunities it would have
afforded, with the rich marine Fauna of the north-eastern coast close at
hand, to develop a large school of Animal Morphology. The existing
Professors at Edinburgh were most desirous that he should join them, and
made every effort to induce him to come. On the part of the Crown, in whose
hands the appointment lay, not only were distinct offers made to him, but
he was repeatedly pressed to accept the post. Nor was it until after a
considerable struggle that he finally refused, his love for his own
University in the end overcoming the many inducements to leave; he elected
to stay where he was, trusting to the future opening up for him some
suitable position. In this decision he was undoubtedly influenced by the
consideration that Cambridge, besides being the centre of his old
friendships, had become as it were a second home for his own family. By the
appointment of Lord Rayleigh to the chair of Experimental Physics his
sister Lady Rayleigh had become a resident, his sister Mrs Sidgwick had
lived there now for some years, and his brother Gerald generally spent the
summer there; their presence made Cambridge doubly dear to him.
At the close of the Michaelmas term, with feelings of relief at having
completed his _Comparative Embryology_, the preparation of the second
volume of which had led to almost incessant labour during the preceding
year, he started to spend the Christmas vacation with his friend
Kleinenberg at Messina. Stopping at Naples on his way thither he found his
pupil Caldwell, who had been sent to occupy the University table at the
Stazione Zoologica, lying ill at Capri, with what proved to be typhoid
fever. The patient was alone, without any friend to tend him, and his
mother who had been sent for had not yet arrived. Accordingly Balfour (with
the kindness all forgetful of himself which was his mark all his life
through) stayed on his journey to nurse the sick man until the mother came.
He then went on to Messina, and there seemed to be in good health, amusing
himself with the ascent of Etna. Yet in January, soon after his return
home, he complained of being unwell, and in due time distinct symptoms of
typhoid fever made their appearance. The attack at first promised to be
severe, but happily the crisis was soon safely passed and the convalescence
was satisfactory.
While yet on his sick bed, a last attempt was made to induce him to accept
the Edinburgh offer, and for the last time he refused. These repeated
offers, and the fact that the dangers of his grave illness had led the
University vividly to realize how much they would lose if Balfour were
taken away from them, encouraged his friends to make a renewed effort to
gain for him some adequate position in the University. This time the
attempt was successful, and the authorities took a step, unusual but
approved of by the whole body of resident members of the University; they
instituted a new Professorship of Animal Morphology, to be held by Balfour
during his life or as long as he should desire, but to terminate at his
death or resignation unless it should be otherwise desirable. Accordingly
in May, 1882, he was admitted into the Professoriate as Professor of Animal
Morphology.
During his illness his lectures had been carried on by his Demonstrator, Mr
Adam Sedgwick, who continued to take his place during the remainder of that
Lent Term and during the ensuing Easter Term. The spring Balfour spent
partly in the Channel Islands with his sister Alice, partly in London with
his eldest brother, but in the course of the Easter Term returned to
Cambridge and resumed his work though not his lectures. His recovery to
health was steady and satisfactory, the only drawback being a swelling over
the shin-bone of one leg, due to a blow on the rocks at Sark; otherwise he
was rapidly becoming strong. He himself felt convinced that a visit to the
Alps, with some mountaineering of not too difficult a kind, would complete
his restoration to health. In this view many of his friends coincided; for
the experience of former years had shewn them what a wonderfully beneficial
effect the Alpine air and exercise had upon his health. He used to go away
pale, thin and haggard, to return bronzed, clear, firm and almost stout;
nor was there anything in his condition which seemed to forbid his
climbing, provided that he was cautious at the outset. Accordingly, early
in June he left Cambridge for Switzerland, having long ago, during his
illness in fact, engaged his old guide, Johann Petrus, whom he had first
met in 1880, and who had always accompanied him in his expeditions since.
His first walking was in the Chamonix district; and here he very soon found
his strength and elasticity come back to him. Crossing over from Montanvert
to Courmayeur, by the Col du Géant, he was attracted by the peak called the
Aiguille Blanche de Peuteret, a virgin peak, the ascent of which had been
before attempted but not accomplished. Consulting with Petrus he determined
to try it, feeling that the fortnight, which by this time he had spent in
climbing, had brought back to him his old vigour, and that his illness was
already a thing of the past.
There is no reason to believe that he regarded the expedition as one of
unusual peril; and an incident which at the time of his death was thought
by some to indicate this was in reality nothing more than a proof of his
kindly foresight. The guide Petrus was burdened by a debt on his land
amounting to about £150. In the previous year Balfour and his brother had
come to know of this debt; and, seeing that no Alpine ascent is free from
danger, that on any expedition some accident might carry them off, had
conceived the idea of making some provision for Petrus' family in case he
might meet with sudden death in their service. This suggestion of the
previous year Balfour carried out on this occasion, and sent home to his
brother Gerald a cheque of £150 for this purpose. But the cheque was sent
from Montanvert before he had even conceived the idea of ascending the
Aiguille Blanche. It was not a provision for any specially dangerous
ascent, and must be regarded as a measure prompted not by a sense of coming
peril but rather by the donor's generous care for his servant.
On Tuesday afternoon, July 18, he and Petrus, with a porter to carry
provisions and firing to their sleeping-place on the rocks, set out from
Courmayeur, the porter returning the same night. They expected to get back
to Courmayeur some time on the Thursday, but the day passed without their
appearing. This did not cause any great anxiety because it was supposed
that they might have found it more convenient to pass over to the Chamonix
side than to return to Courmayeur. When on Friday however telegrams
dispatched to Chamonix and Montanvert brought answers that nothing had been
seen of them, it became evident that some accident had happened, and an
exploring party set out for the hills. It was not until early on the Sunday
morning that this search party found the bodies, both partly covered with
snow, lying on the Glacier de Fresney, below the impassable icefall which
separates the upper basin of the glacier from the lower portion, and at the
foot of a _couloir_ which descends by the side of the icefall. Their tracks
were visible on the snow at the top of the _couloir_. Balfour's neck was
broken, and his skull fractured in three places; Petrus' body was also
fractured in many places. The exact manner of their death will never be
known, but there can be no doubt that, in Balfour's case at all events, it
was instantaneous, and those competent to form a judgment are of opinion
that they were killed by a sudden fall through a comparatively small
height, slipping on the rocks as they were descending by the side of the
ice-fall, and not precipitated from the top of the _couloir_. There is
moreover indirect evidence which renders it probable that in the fatal fall
Petrus slipped first and carried Balfour with him. Whether they had reached
the summit of the Aiguille and were returning home after a successful
ascent or whether they were making their way back disheartened and wearied
with failure, is not and perhaps never will be known. Since the provisions
at the sleeping-place were untouched, the deaths probably took place on
Wednesday the 19th. The bringing down the bodies proved to be a task of
extreme difficulty, and it was not till Wednesday the 26th that the remains
reached Courmayeur, where M. Bertolini, the master of the hotel, and indeed
everyone, not least the officers of a small body of Italian troops
stationed there, shewed the greatest kindness and sympathy to Balfour's
brothers, Gerald and Eustace, who hastened to the spot as soon as the news
of the terrible disaster was telegraphed home. Mr Walter Leaf also and Mr
Conway, friends of Balfour, the former a very old one, who had made their
way to Courmayeur from other parts of Switzerland as soon as they heard of
the accident, rendered great assistance. The body was embalmed, brought to
England, and buried at Whittinghame on Saturday, Aug. 5, the Fellows of
Trinity College holding a service in the College Chapel at the same time.
In person he was tall, being fully six feet in height, well built though
somewhat spare. A broad forehead overhanging deeply set dark brown eyes
whose light shining from beneath strongly marked eye-brows told all the
changes of his moods, slightly prominent cheek-bones, a pale skin, at times
inclined to be even sallow, dark brown hair, allowed to grow on the face
only as a small moustache, and slight whiskers, made up a countenance which
bespoke at once strength of character and delicacy of constitution. It was
an open countenance, hiding nothing, giving sign at once, both when his
body was weary or weak, and when his mind was gladdened, angered or
annoyed.
The record of some of his thoughts and work, all that he had given to the
world will be found in the following pages. But who can tell the ideas
which had passed into his quick brain, but which as yet were known only to
himself, of which he had given no sign up to that sad day on which he made
the fatal climb? And who can say whither he might not have reached had he
lived, and his bright young life ripened as years went on? This is not the
place to attempt any judgment of his work: that may be left to other times,
and to other hands; but it may be fitting to place here on record a letter
which shews how much the greatest naturalist of this age appreciated his
younger brother. Among Balfour's papers was found a letter from Charles
Darwin, acknowledging the receipt of Vol. II. of the _Comparative
Embryology_ in the following words:
"_July 6, 1881._
DOWN, BECKENHAM, KENT.
MY DEAR BALFOUR,
I thank you heartily for the present of your grand book, and
I congratulate you on its completion. Although I read almost
all of Vol. I, I do not feel that I am worthy of your
present, unless indeed the fullest conviction that it is a
memorable work makes me worthy to receive it.
* * * * *
Once again accept my thanks, for I am proud to receive a
book from you, who, I know, will some day be the chief of
the English Biologists.
Believe me,
Yours sincerely,
CHARLES DARWIN."
The loss of him was a manifold loss. He is mourned, and will long be
mourned, for many reasons. Some miss only the brilliant investigator;
others feel that their powerful and sympathetic teacher is gone; some look
back on his memory and grieve for the charming companion whose kindly
courtesy and bright wit made the hours fly swiftly and pleasantly along;
and to yet others is left an aching void when they remember that they can
never again lean on the friend whose judgment seemed never to fail and
whose warm-hearted affection was a constant help. And to some he was all of
these. At the news of his death the same lines came to the lips of all of
us, so fittingly did Milton's words seem to speak our loss and grief--
"For Lycidas is dead, dead ere his prime,
Young Lycidas, and hath not left his peer."
M. FOSTER.
I. ON SOME POINTS IN THE GEOLOGY OF THE EAST LOTHIAN COAST[3].
Footnote 3: From the _Geological Magazine_, Vol. IX. No. 4.
April, 1872.
By G. W. and F. M. BALFOUR, Trinity College, Cambridge.
The interesting relation between the Porphyrite of Whitberry Point, at the
mouth of the Tyne, near Dunbar, and the adjacent sedimentary rocks, was
first noticed, we believe, by Professor Geikie, who speaks of it in the
_Memoirs of the Geological Survey of East Lothian_, pages 40 and 31, and
again in the new edition of Jukes's _Geology_, p. 269. The volcanic mass
which forms the point consists of a dark felspathic base with numerous
crystals of augite: it is circular in form, and is exposed for two-thirds
of its circumference in a vertical precipice facing the sea, about twenty
feet in height.
The rock is traversed by numerous joints running both in a horizontal and
in a vertical direction. The latter are by far the most conspicuous, and
give the face of the cliff, when seen from a distance, a well-marked
columnar appearance, though the columns themselves are not very distinct or
regular. They are quadrangular in form, and are evidently produced by the
intersection at right-angles of the two series of vertical joints.
It is clear that the face of the precipice has been gradually receding in
proportion as it yielded to the action of the waves; and that at a former
period the volcanic rock extended considerably further than at present over
the beds which are seen to dip beneath it. These latter consist of hard
fine-grained calcareous sandstones belonging to the Lower Carboniferous
formation. Their colour varies from red to white, and their prevailing dip
is in a N.W. direction, with an average inclination of 12-20°. If the
volcanic mass is a true intrusive rock, we should naturally expect the
strata which surround it to dip _away_ in all directions, the amount of
their inclination diminishing in proportion to their distance from it. We
find, however, that the case is precisely the reverse: as the beds approach
the base of the cliff, they dip _towards_ it from every side at perpetually
increasing angles, until at the point of junction the inclination amounts
in places to as much as 55 degrees. The exact amount of dip in the various
positions will be seen on referring to the accompanying map.
[Illustration: FIG. 1. MAP OF STRATA AT WHITBERRY POINT. Scale, 6 in. to
the mile.
_A._ Lava sheet. _B._ Sandstone Beds, dipping from every side towards the
lava. _CC._ Line of Section along which Fig. 2 is supposed to be drawn.]
We conceive that the phenomenon is to be explained by supposing the orifice
through which the lava rose and overflowed the surface of the sedimentary
strata to have been very much smaller in area than the extent of igneous
rock at present visible; and that the pressure of the erupted mass on the
soft beds beneath, aided perhaps by the abstraction of matter from below,
caused them to incline towards the central point at a gradually increasing
angle. The diagram, fig. 2, will serve further to illustrate this
hypothesis. _A_ is the neck or orifice by which the melted matter is
supposed to ascend. _C_ shews the sheet of lava after it has overspread the
surface of the sandstone beds _B_, so as to cause them to assume their
present inclination. The dotted lines represent the hypothetical extension
of the igneous mass and sandstones previous to the denudation which they
have suffered from the action of the waves.
Professor Geikie, in his admirable treatise on the Geology of the
county[4], adopts a view on this subject which is somewhat different from
that which is suggested in this paper. He considers that the whole mass is
an intrusive neck of rock with perpendicular sides; and that it once filled
up an orifice through the surrounding sedimentary strata, of which it is
now the only remnant.
Footnote 4: _Memoirs of Geological Survey of Scotland_, sheet
33, pp. 40, 41.
[Illustration: FIG. 2. VERTICAL SECTION THROUGH CC. DIAGRAM (FIG. 1).
_A._ Orifice by which the lava ascended. _B._ Sandstone Beds. _B´._
Hypothetical extension of ditto. _C._ Sheet of lava spread over the
sandstones _B_. _C´_. Hypothetical extension of ditto.]
He admits that the inclination of the sandstone beds towards the igneous
mass in the centre is a phenomenon that is somewhat difficult to explain,
and suggests that a subsequent contraction of the column may have tended to
produce such a result. To use his own words: "In the case of a solid column
of felstone or basalt, the contraction of the melted mass on cooling may
have had some effect in dragging down the sides of the orifice[5]."
Footnote 5: Note on p. 41 of _Mem. Geol. Survey of East
Lothian_.
But, apart from other objections, it is scarcely conceivable that this
result should have been produced by the contraction of the column.
In his recent edition of Jukes's _Manual of Geology_ (p. 269), in which he
also refers to this instance, he states that in other cases of "necks" it
is found to be an almost invariable rule, "that strata are bent down so as
to dip into the neck all round its margin." We are not aware to what other
instances Prof. Geikie may allude; but on referring to his _Memoir on the
Geology of East Lothian_, we find that he states in the cases of 'North
Berwick Law' and 'Traprain' (which he compares with the igneous mass at
Whitberry Point), that the beds at the base of these two necks, where
exposed, dip _away_ from them, and that at a high angle.
In support of the hypothesis which we have put forward, the following
arguments may be urged:
(1) That in one place at least the sedimentary strata are seen to be
actually dipping beneath the superincumbent basalt; and that the impression
produced by the general relation of the two rocks is, that they do so
everywhere.
(2) Since the columns into which the lava is split are vertical, the
cooling surface must have been horizontal: the mass must, therefore, have
formed a sheet, and not a dyke; for, in the latter case, the cooling
surfaces would have been vertical.
(3) It is difficult to conceive, on the supposition that the volcanic rock
is a neck with perpendicular sides, that the marine denudation should have
uniformly proceeded only so far as to lay bare the junction between the two
formations. We should have expected that in many places the igneous rock
itself would have been cut down to the general level, whereas the only
signs of such an effect are shown in a few narrow inlets where the rock was
manifestly softer than in the surrounding parts.
The last objection is greatly confirmed by the overhanging cliffs and
numerous blocks of porphyrite which lie scattered on the beach, as if to
attest the former extension of that ancient sheet of which these blocks now
form but a small remnant. Indeed, the existence of such remains appears
sufficient of itself to condemn any hypothesis which presumes the present
face of the cliff to have formed the original boundary of the mass.
It may be fairly objected to our theory, as Prof. Geikie himself has
suggested, that the high angle at which the strata dip is difficult to
account for. But, in fact, this steep inclination constitutes the very
difficulty which any hypothesis on the subject must be framed to explain;
and it is a difficulty which is not more easily solved by Prof. Geikie's
theory than by our own.
II. THE DEVELOPMENT AND GROWTH OF THE LAYERS OF THE BLASTODERM[6].
Footnote 6: From the _Quarterly Journal of Microscopical
Science_, Vol. XIII., 1873.
With Plate 1, figs. 1-5 and 9-12.
The following paper deals with the changes which take place in the cells of
the blastoderm of the hen's egg during the first thirty or forty hours of
incubation. The subject is one which has, as a general rule, not been much
followed up by embryologists, but is nevertheless of the greatest interest,
both in reference to embryology itself, and to the growth and changes of
protoplasm exhibited in simple embryonic cells. I am far from having
exhausted the subject in this paper, and in some cases I shall be able
merely to state facts, without being able to give any explanation of their
meaning.
My method of investigation has been the examination of sections and surface
views. For hardening the blastoderm I have employed, as usual, chromic
acid, and also gold chloride. It is, however, difficult to make sections of
blastoderms hardened by this latter reagent, and the sections when made are
not in all cases satisfactory. For surface views I have chiefly used silver
nitrate, which brings out the outlines of the cells in a manner which
leaves nothing to be desired as to clearness. If the outlines only of the
cells are to be examined, a very short immersion (half a minute) of the
blastoderm in a half per cent. solution of silver nitrate is sufficient,
but if the immersion lasts for a longer period the nuclei will be brought
out also. For studying the latter, however, I have found it better to
employ gold chloride or carmine in conjunction with the silver nitrate.
My observations begin with the blastoderm of a freshly laid egg. The
appearances presented by sections of this have been accurately described by
Peremeschko, "Ueber die Bildung der Keimblätter im Hühnerei,"
_Sitzungsberichte der K. Akademie der Wissenschaften in Wien_, 1868.
Oellacher, "Untersuchung über die Furchung und Blatterbildung im Hühnerei,"
_Studien aus dem Institut für Experim. Pathologie in Wien_, 1870 (pp.
54-74), and Dr Klein, lxiii. Bande der _Sitz. der K. Acadamie der Wiss. in
Wien_, 1871.
The unincubated blastoderm (Pl. 1, fig. 1) consists of two layers. The
upper layer is composed of a single row of columnar cells. Occasionally,
however, the layer may be two cells thick. The cells are filled with highly
refracting spherules of a very small size, and similar in appearance to the
finest white yolk spherules, and each cell also contains a distinct oval
nucleus. This membrane rests with its extreme edge on the white yolk, its
central portion covering in the segmentation cavity. From the very first it
is a distinct coherent membrane, and exhibits with silver nitrate a
beautiful hexagonal mosaic of the outlines (Pl. 1, fig. 6) of the cells.
The diameter of the cells when viewed from above is from 1/2000 - 1/3000 of
an inch. The under layer is very different from this: it is composed of
cells which are slightly, if at all, united, and which vary in size and
appearance, and in which a nucleus can rarely be seen. The cells of which
it is composed fill up irregularly the segmentation cavity, though a
distinct space is even at this time occasionally to be found at the bottom
of it. Later, when the blastoderm has spread and the white yolk floor has
been used as food, a considerable space filled with fluid may generally be
found.
The shape of the floor of the cavity varies considerably, but it is usually
raised in the middle and depressed near the circumference. In this case the
under layer is perhaps only two cells deep at the centre and three or four
cells deep near the circumference.
The cells of which this layer is composed vary a good deal in size; the
larger cells being, however, more numerous in the lower layers. In
addition, there are usually a few very large cells quite at the bottom of
the cavity, occasionally separated from the other cells by fluid. They were
called _formative cells_ (Bildungselemente) by Peremeschko (_loc. cit._);
and, according to Oellacher's observations (_loc. cit._), some of them, at
any rate, fall to the bottom of the segmentation cavity during the later
stages of segmentation. They do not differ from the general lower layer
cells except in size, and even pass into them by insensible gradations. All
the cells of the lower layer are granular, and are filled with highly
refracting spherules precisely similar to the smaller white yolk spherules
which line the bottom of the segmentation cavity.
The size of the ordinary cells of the lower layer varies from 1/2000 -
1/1000 of an inch. The largest of the formative cells come up to 1/300 of
an inch. It will be seen from this description that, morphologically
speaking, we cannot attach much importance to the formative cells. The fact
that they broke off from the blastoderm, towards the end of the
segmentation--even if we accept it as a normal occurrence, rather than the
result of manipulation--is not of much importance, and, except in size, it
is impossible to distinguish these cells from other cells of the lower
layer of the blastoderm.
Physiologically, however, as will be afterwards shewn, they are of
considerable importance.
The changes which the blastoderm undergoes during the first three or four
hours of incubation are not very noticeable. At about the sixth or eighth
hour, or in some cases considerably earlier, changes begin to take place
very rapidly. These changes result in the formation of a hypoblast and
mesoblast, the upper layer of cells remaining comparatively unaltered as
the epiblast.
To form the hypoblast a certain number of the cells of the lower layer
begin to undergo remarkable changes. From being spherical and, as far as
can be seen, non-nucleated, they become (vide fig. 2, _h_) flattened and
nucleated, still remaining granular, but with fewer spherules.
Here, then, is a direct change, of which all the stages can be followed, of
a cell of one kind into a cell of a totally different character. The new
cell is not formed by a destruction of the old one, but directly from it by
a process of metamorphosis. These hypoblast cells are formed first at the
centre and later at the circumference, so that from the first the cells at
the circumference are less flattened and more granular than the cells at
the centre. A number of cells of the original lower layer are enclosed
between this layer and the epiblast; and, in addition to these, the
formative cells (as has been shewn by Peremeschko, Oellacher, and Klein,
whose observations I can confirm) begin to travel towards the
circumference, and to pass in between the epiblast and hypoblast.
Both the formative cells, and the lower layer cells enclosed between the
hypoblast and epiblast, contribute towards the mesoblast, but the mode in
which the mesoblast is formed is very different from that in which the
hypoblast originates.
It is in this difference of formation that the true distinction between the
mesoblast and hypoblast is to be looked for, rather than in the original
difference of the cells from which they are derived.
The cells of the mesoblast are formed by a process which seems to be a kind
of free cell formation. The whole of the interior of each of the formative
cells, and of the other cells which are enclosed between the epiblast and
the hypoblast, become converted into new cells. These are the cells of the
mesoblast. I have not been able perfectly to satisfy myself as to the exact
manner in which this takes place, but I am inclined to think that some or
all of the spherules which are contained in the original cells develop into
nuclei for the new cells, the protoplasm of the new cells being formed from
that of the original cells.
The stages of formation of the mesoblast cells are shewn in the section
(Pl. 1, fig. 2), taken from the periphery of a blastoderm of eight hours.
The first formation of the mesoblast cells takes place in the centre of the
blastoderm, and the mass of cells so formed produces the opaque line known
as the primitive streak. This is shown in Pl. 1, fig. 9.
One statement I have made in the above description in reference to the
origin of the mesoblast cells, viz. that they are only partly derived from
the formative cells at the bottom of the segmentation cavity, is to a
certain extent opposed to the statements of the three investigators above
mentioned. They state that the mesoblast is entirely derived from the
formative cells. It is not a point to which I attach much importance,
considering that I can detect no difference between these cells and any
other cells of the original lower layer except that of size; and even this
difference is probably to be explained by their proximity to the white
yolk, whose spherules they absorb. But my reason for thinking it probable
that these cells alone do not form the mesoblast are: 1st. That the
mesoblast and hypoblast are formed nearly synchronously, and except at the
centre a fairly even sprinkling of lower layer cells is from the first to
be distinguished between the epiblast and hypoblast. 2nd. That if some of
the lower layer cells are not converted into mesoblast, it is difficult to
see what becomes of them, since they appear to be too numerous to be
converted into the hypoblast alone. 3rd. That the chief formation of
mesoblast at first takes place in the centre, while if the formative cells
alone took part in its formation, it would be natural to expect that it
would begin to be formed at the periphery.
Oellacher himself has shewn (_Zeitschrift für wissenschaftliche Zoologie_,
1873, "Beiträge zur Entwick. Gesch. der Knochenfische") that in osseous
fishes the cells which break away from the blastoderm take no share in the
formation of the mesoblast, so that we can derive no argument from the
formation of the mesoblast in these animals, for believing that in the
chick it is derived only from the formative cells.
In the later stages, however, from the twelfth to the twenty-fifth hour,
the growth of the mesoblast depends almost entirely on these cells, and
Peremeschko's discovery of the fact is of great value.
Waldeyer (_Henle und v. Pfeufer's Zeitschrift_, xxxiv. Band, für 1869) has
given a different account of the origin of the layers. There is no doubt,
however, in opposition to his statements and drawings, that from the very
first the hypoblast is distinct from the mesoblast, which is, indeed, most
conspicuously shewn in good sections; and his drawings of the derivation of
the mesoblast from the epiblast are not very correct.
The changes which have been described are also clearly shewn by means of
silver nitrate. Whereas, at first this reagent brought out no outline
markings of cells in the lower layer, by the eighth to the twelfth hour the
markings (Pl. 1, fig. 3) are very plain, and shew that the hypoblast is a
distinct coherent membrane.
In section, the cells of the hypoblast appear generally very thin and
spindle shaped, but the outlines brought out by the silver nitrate shew
that they are much expanded horizontally, but very irregular as to size,
varying even within a small area from 1/4000 - 1/400 of an inch in the
longest diameter.
At about the twelfth hour they are uniformly smaller a short way from each
extremity of its longer axis than over the rest of the blastoderm.
It is, perhaps, fair to conclude from this that growth is most rapid at
these parts.
At this time the hypoblast, both in sections and from a surface view after
treatment with silver nitrate, appears to end abruptly against the white
yolk. The surface view also shews that its cells are still filled with
highly refractive globules, making it difficult to see the nucleus. In some
cases I thought that I could (fig. 3, _a_) make out that it was hour-glass
shaped, and some cells certainly contain two nuclei. Some of the cells
(fig. 3, _b_) shew re-entrant curves, which prove that they have undergone
division.
The cells of the epiblast, up to the thirteenth hour, have chiefly
undergone change in becoming smaller.
In surface views they are about 1/4000 of an inch in diameter over the
centre of the pellucid area, and increase to 1/2000 of an inch over the
opaque area.
In the centre of the pellucid area the form of the epiblast cells is more
elongated vertically and over the opaque area more flattened than was the
case with the original upper layer cells. In the centre the epiblast is two
or three cells deep.
Before going on to the further changes of the blastodermic cells it will be
well to say a few words in reference to the origin of the mesoblast.
From the description given above it will be clear that in the chick the
mesoblast has an independent origin; it can be said neither to originate
from the epiblast nor from the hypoblast. It is formed coincidently with
the latter out of apparently similar segmentation cells. The hypoblast, as
has been long known, shews in the chick no trace of its primitive method of
formation by involution, neither does the mesoblast shew any signs of its
primitive mode of formation. In so excessively highly differentiated a type
as birds we could hardly expect to find, and certainly do not find, any
traces of the primitive origin of the mesoblast, either from the epiblast
or hypoblast, or from both. In the chick the mesoblast cells are formed
directly from the ultimate products of segmentation. From having a
secondary origin in most invertebrates the mesoblast comes to have, in the
chick, a primary origin from the segmentation spheres, precisely as we find
to be the case with the nervous layer in osseous fishes. It is true we
cannot tell which segmentation-cells will form the mesoblast, and which the
hypoblast; but the mesoblast and hypoblast are formed at the same time, and
both of them directly from segmentation spheres.
The process of formation of the mesoblast in Loligo, as observed by Mr Ray
Lankester (_Annals and Magazine of Natural History_, February, 1873), is
still more modified. Here the mesoblast arises independently of the
blastoderm, and by a process of free cell-formation in the yolk round the
edge of the blastoderm. If Oellacher's observations in reference to the
origin of formative cells are correct, then the modes of origin of the
mesoblast in Loligo and the chick would have nothing in common; but if the
formative cells are in reality derived from the white yolk, and also are
alone concerned in the formation of the mesoblast, then the modes of
formation of the mesoblast in the chick would be substantially the same as
that observed by Mr Ray Lankester in Loligo.
No very important changes take place in the actual forms of the cells
during the next few hours. A kind of fusion takes place between the
epiblast and the mesoblast along the line of the primitive streak forming
the axis-string of His; but the line of junction between the layers is
almost always more or less visible in sections. In any case it does not
appear that there is any derivation of mesoblast cells from the epiblast;
and since the fusion only takes place in the region of the primitive
groove, and not in front, where the medullary groove arises (see succeeding
paper), it cannot be considered of any importance in reference to the
possible origin of the Wolffian duct, &c., from the epiblast (as mooted by
Waldeyer, _Eierstock und Ei_, Leipzig, 1870). The primitive groove, as can
be seen in sections, begins to appear very early, generally before the
twelfth hour. The epiblast spreads rapidly over the white yolk, and the
area pellucida also increases in size.
From the mesoblast forming at first only a small mass of cells, which lies
below the primitive streak, it soon comes to be the most important layer of
the blastoderm. Its growth is effected by means of the formative cells.
These cells are generally not very numerous in an unincubated blastoderm,
but rapidly increase in numbers, probably by division; at the same time
they travel round the edge of, and in some cases through, the hypoblast,
and then become converted in the manner described into mesoblast cells.
They act as carriers of food from the white yolk to the mesoblast till,
after the formation of the vascular area, they are no longer necessary. The
numerous cases in which two nucleoli and even two nuclei can be seen in one
cell prove that the mesoblast cells also increase by division.
The growth of the hypoblast takes place in a very different way. It occurs
by a direct conversion, cell for cell, of the white yolk spheres into
hypoblast cells. This interpretation of the appearances, which I will
describe presently, was first suggested to me by Dr Foster, from an
examination of some of my specimens of about thirty-six hours, prepared
with silver nitrate. Where there is no folding at the junction between the
pellucid and opaque areas, there seems to be a perfect continuity in the
silver markings and a gradual transition in the cells, from what would be
undoubtedly called white yolk spheres, to as undoubted hypoblast cells
(vide Pl. 1, fig. 5). In passing from the opaque to the pellucid areas the
number of white yolk spherules in each cell becomes less, but it is not
till some way into the pellucid area that they quite cease to be present. I
at first thought that this was merely due to the hypoblast cells feeding on
the white yolk sphericles, but the perfect continuity of the cells, and the
perfect gradation in passing from the white yolk cells to the hypoblast,
proves that the other interpretation is the correct one, viz. that the
white yolk spheres become directly converted into the hypoblast cells. This
is well shewn in sections (vide Pl. 1, fig. 4) taken from embryos of all
ages from the fifteenth to the thirty-sixth hour and onwards. But it is,
perhaps, most easily seen in embryos of about twenty hours. In such an
embryo there is a most perfect gradation: the cells of the hypoblast
become, as they approach the edge of the pellucid area, broader, and are
more and more filled with white yolk sphericles, till at the line of
junction it is quite impossible to say whether a particular cell is a
white-yolk cell (sphere) or a hypoblast cell. The white-yolk cells near the
line of junction can frequently be seen to possess nuclei. At first the
hypoblast appears to end abruptly against the white yolk; this state of
things, however, soon ends, and there supervenes a complete and unbroken
continuity between the hypoblast and the white yolk.
Of the mode of increase of the epiblast I have but little to say. The cells
undoubtedly increase entirely by division, and the new material is most
probably derived directly from the white yolk.
Up to the sixth hour the cells of the upper layer retain their early
regular hexagonal pattern, but by the twelfth hour they have generally
entirely lost this, and are irregularly shaped and very angular. The cells
over the centre of the pellucid area remain the smallest up to the
twenty-fifth hour or later, while those over the rest of the pellucid area
are uniformly larger.
In the hypoblast the cells under the primitive groove, and on each side as
far as the fold which marks off the exterior limit of the protovertebræ are
at the eighteenth hour considerably smaller than any other cells of this
layer.
In all the embryos between the eighteenth and twenty-third hour which I
have examined for the purpose, I have found that at about two-thirds of the
distance from the anterior end of the pellucid area, and just external to
the side fold, there is a small space on each side in which the cells are
considerably larger than anywhere else in the hypoblast. These larger
cells, moreover, contain a greater number of highly refractive spherules
than any other cells. It is not easy to understand why growth should have
been less rapid here than elsewhere, as the position does not seem to
correspond to any feature in the embryo. In some specimens the hypoblast
cells at the extreme edge of the pellucid area are smaller than the cells
immediately internal to them. At about the twenty-third hour these cells
begin rapidly to lose the refractive spherules they contained in the
earlier stages of incubation, and come to consist of a nucleus surrounded
simply by granular protoplasm.
At about this period of incubation the formative cells are especially
numerous at the periphery of the blastoderm, and, no doubt, become
converted into the mass of mesoblast which is found at about the
twenty-fifth hour in the region of the vascular area. Some of them are
lobate, and appear as if they were undergoing division. At this time also
the greatest number of formative cells are to be found at the bottom of the
now large segmentation cavity.
In embryos of from thirty to forty hours the cells of the hypoblast have,
over the central portion of the pellucid area, entirely lost their highly
refractive spherules, and in the fresh state are composed of the most
transparent protoplasm. When treated with reagents they are found to
contain an oval nucleus with one or sometimes two nucleoli, imbedded in a
considerable mass of protoplasm. The protoplasm appears slightly granular
and generally contains one or two small vacuoles. I have already spoken of
the gradation of the hypoblast at the edge of the blastoderm into white
yolk. I have, therefore, only to mention the variations in the size of its
cells in different parts of the pellucid area. The points where the cells
are smallest seem generally to coincide with the points of maximum growth.
Over the embryo the cells are more regular than elsewhere. They are
elongated and arranged transversely to the long axis of the embryo. They
are somewhat hexagonal in shape, and not unlike the longer pieces in the
dental plate of a Myliobatis (Pl. 1, fig. 10). This regularity, however, is
much more marked in some specimens than in others. These cells are about
1/4000th of an inch in breadth, and 1/1000th in length. On each side of the
embryo immediately external to the protovertebræ the cells are frequently
about the same size as those over the embryo itself. In the neck, however,
and near the end of the sinus rhomboidalis, they are considerably smaller,
about 1/4000th inch each way. The reason of this small size is not very
clear, but probably shews that the greatest growth is taking place at these
two points. The cells, again, are very small at the head fold, but are very
much larger in front of this--larger, in fact, than any other cells of the
hypoblast. Outside the embryo they gradually increase in size towards the
edge of the pellucid area. Here they are about 1/1000th of an inch in
diameter, irregular in shape and rather angular.
The outlines of the cells of the epiblast at this time are easily
distinguished from the cells of the hypoblast by being more elongated and
angular; they are further distinguished by the presence of numerous small
oval cells, frequently at the meeting point of several cells, at other
times at points along the lines of junction of two cells (Pl. 1, fig. 12).
These small cells look very like the smaller stomata of endothelial
membranes, but are shewn to be cells by possessing a nucleus. There is
considerable variation in size in the cells in different parts of the
epiblast. Between the front lobes of the brain the cells are very small,
1/4000th inch, rising to 1/2000th on each side. They are about the latter
size over the greater part of the embryo. But over the sinus rhomboidalis
they fall again to from 1/3000th to 1/4000th inch. This is probably to be
explained by the growth of the medullary fold at this point, which pushes
back the primitive groove. At the sides of the head the cells are larger
than anywhere else in the epiblast, being here about 1/1000th inch in
diameter. I at present see no explanation of this fact. At the periphery of
the pellucid area and over the vascular area the cells are 1/1500th to
1/2000th inch in diameter, but at the periphery of the opaque area they are
smaller again, being about the 1/3000th of an inch. This smaller size at
the periphery of the area opaca is remarkable, since in the earlier stages
the most peripheral epiblast cells were the largest. It, perhaps, implies
that more rapid growth is at this time taking place in that part of the
epiblast which is spreading over the yolk sac.
EXPLANATION OF PLATE 1, Figs. 1-5 and 9-12.
Fig. 1. Section through an unincubated blastoderm, shewing the upper layer,
composed of a single row of columnar cells, and the lower layer, composed
of several rows of rounded cells in which no nucleus is visible. Some of
the "formative cells," at the bottom of the segmentation cavity, are seen
at (_b_).
Fig. 2. Section through the periphery of an eight hours' blastoderm,
shewing the epiblast (_p_), the hypoblast (_h_), and the mesoblast
commencing to be formed (_c_), partly by lower-layer cells enclosed between
the epiblast and hypoblast, and partly by formative cells. Formative cells
at the bottom of the segmentation cavity are seen at _b_. At _s_ is one of
the side folds parallel to the primitive groove.
Fig. 3. Portion of the hypoblast of a thirteen hours' blastoderm, treated
with silver nitrate, shewing the great variation in the size of the cells
at this period. An hour-glass shaped nucleus is seen at _a_.
Fig. 4. Periphery of a twenty-three hours' blastoderm, shewing cell for
cell the junction between the hypoblast (_h_) and white-yolk spheres (_w_).
Fig. 5. Junction between the white-yolk spheres and the hypoblast cells at
the passage from the area pellucida to the area opaca. The specimen was
treated with silver nitrate to bring out the shape of the cells. The line
of junction between the opaque and pellucid areas passes diagonally.
Fig. 9. Section through the primitive streak of an eight hours' blastoderm.
The specimen shews the mesoblast very much thickened in the immediate
neighbourhood of the primitive streak, but hardly formed at all on each
side of the streak. It also shews the primitive groove just beginning to be
formed (_pr_), and the fusion between the epiblast and the mesoblast under
the primitive groove. The hypoblast is completely formed in the central
part of the blastoderm. At _f_ is seen one of the side folds parallel to
the primitive groove. Its depth has been increased by the action of the
chromic acid.
Fig. 10. Hypoblast cells from the hinder end of a thirty-six hours' embryo,
treated with silver nitrate, shewing the regularity and elongated shape of
the cells over the embryo and the smaller cells on each side.
Fig. 11. Epiblast cells from an unincubated blastoderm, treated with silver
nitrate, shewing the regular hexagonal shape of the cells and the small
spherules they contain.
Fig. 12. Portion of the epiblast of a thirty-six hours' embryo, treated
with silver nitrate, shewing the small rounded cells frequently found at
the meeting-points of several larger cells which are characteristic of the
upper layer.
III. ON THE DISAPPEARANCE OF THE PRIMITIVE GROOVE IN THE EMBRYO CHICK[7].
Footnote 7: From the _Quarterly Journal of Microscopical
Science_, Vol. XIII, 1873.
With Plate 1, Figs. 6-8 and 13-19.
The investigations of Dursy (_Der Primitivstreif des Hühnchens_, von Dr E.
Dursy. Lahr, 1866) on the primitive groove, shewing that it is a temporary
structure, and not connected with the development of the neural canal, have
in this country either been ignored or rejected. They are, nevertheless,
perfectly accurate; and had Dursy made use of sections to support his
statements I do not think they would so long have been denied. In Germany,
it is true, Waldeyer has accepted them with a few modifications, but I have
never seen them even alluded to in any English work. The observations which
I have made corroborating Dr Dursy may, perhaps, under these circumstances
be worth recording.
After about twelve hours of incubation the pellucid area of a hen's egg has
become somewhat oval, with its longer axis at right angles to the long axis
of the egg. Rather towards the hinder (narrower) end of this an opaque
streak has appeared, with a somewhat lighter line in the centre. A section
made at the time shews that the opaque streak is due partly to a thickening
of the epiblast, but more especially to a large collection of the rounded
mesoblast cells, which along this opaque line form a thick mass between the
epiblast and the hypoblast. The mesoblast cells are in contact with both
hypoblast and epiblast, and appear to be fused with the latter. The line of
junction between them can, however, almost always be made out.
Soon after the formation of this primitive streak a groove is formed along
its central line by a pushing inwards of the epiblast. The epiblast is not
thinner where it lines the groove, but the mass of mesoblast below the
groove is considerably thinner than at its two sides. This it is which
produces the peculiar appearance of the primitive groove when the
blastoderm is viewed by transmitted light as a transparent line in the
middle of an opaque one.
This groove, as I said above, is placed at right angles to the long axis of
the egg, and nearer the hind end, that is, the narrower end of the pellucid
area. It was called "the primitive groove" by the early embryologists, and
they supposed that the neural canal arose from the closure of its edges
above. It is always easy to distinguish this groove, in transverse
sections, by several well-marked characters. In the first place, the
epiblast and mesoblast always appear more or less fused together underneath
it; in the second place, the epiblast does not become thinner where it
lines the groove; and in the third place, the mesoblast beneath it never
shews any signs of being differentiated into any organ.
As Dursy has pointed out, there is frequently to be seen in fresh
specimens, examined as transparent objects, a narrow opaque line running
down the centre of this groove. I do not know what this line is caused by,
as there does not appear to be any structural feature visible in sections
to which it can correspond.
From the twelfth to the sixteenth hour the primitive groove grows rapidly,
and by the sixteenth hour is both absolutely and considerably longer than
it was at the twelfth hour, and also proportionately longer as compared
with the length of the pellucid area.
There is a greater interval between its end and that of the pellucid area
in front than behind.
At about the sixteenth hour, or a little later, a thickening of the
mesoblast takes place in front of the primitive groove, forming an opaque
streak, which in fresh specimens looks like a continuation from the
anterior extremity of the primitive groove (vide Pl. 1, fig. 8). From
hardened specimens, however, it is easy to see that the connection of this
streak with the primitive groove is only an apparent one. Again, it is
generally possible to see that in the central line of this streak there is
a narrow groove. I do not feel certain that there is no period when this
groove may not be present, but its very early appearance has not been
recognized either by Dursy or by Waldeyer. Moreover, both these authors, as
also His, seem to have mistaken the opaque streak spoken of above for the
notochord. This, however, is not the case, and the notochord does not make
its appearance till somewhat later. The mistake is of very minor
importance, and probably arose in Dursy's case from his not sufficiently
making use of sections. At about the time the streak in front of the
primitive groove makes its appearance a semicircular fold begins to be
formed near the anterior extremity of the pellucid area, against which the
opaque streak, or as it had, perhaps, better be called, "the medullary
streak," ends abruptly.
This fold is the head fold, and the groove along the medullary streak is
the medullary groove, which subsequently forms the cavity of the medullary
or neural canal.
Everything which I have described above can without difficulty be made out
from the examination of fresh and hardened specimens under the simple
microscope; but sections bring out still more clearly these points, and
also shew other features which could not have been brought to light without
their aid. In Pl. 1, figs. 6 and 7, two sections of an embryo of about
eighteen hours are shewn. The first of these passes through the medullary
groove, and the second of them through the extreme anterior end of the
primitive groove. The points of difference in the two sections are very
obvious.
From fig. 6 it is clear that a groove has already been formed in the
medullary streak, a fact which was not obvious in the fresh specimen. In
the second place the mesoblast is thickened both under the groove and also
more especially in the medullary folds at the sides of the groove; but
shews hardly a sign of the differentiation of the notochord. So that it is
clear that the medullary streak is not the notochord, as was thought to be
the case by the authors above mentioned. In the third place there is no
adhesion between the epiblast and the mesoblast. In all the sections I have
cut through the medullary groove I have found this feature to be constant;
while (for instance, as in Pl. 1, figs. 7, 9, 17) all sections through the
primitive groove shew most clearly an adhesion between the epiblast and
mesoblast. This fact is both strongly confirmatory of the separate origins
of the medullary and primitive grooves, and is also important in itself, as
leaving no loophole for supposing that in the region of embryo there is any
separation of the cells from the epiblast to form the mesoblast.
By this time the primitive groove has attained its maximum growth, and from
this time begins both absolutely to become smaller, and also gradually to
be pushed more and more backwards by the growth of the medullary groove.
The specimen figured in Pl. 1, fig. 18, magnified about ten diameters,
shews the appearance presented by an embryo of twenty-three hours. The
medullary groove (_mc_) has become much wider and deeper than it was in the
earlier stage; the medullary folds (_A_) are also broader and more
conspicuous. The medullary groove widens very much posteriorly, and also
the medullary folds separate far apart to enclose the anterior end of the
primitive groove (_pr_).
All this can easily be seen with a simple microscope, but the sections
taken from the specimen figured also fully bear out the interpretations
given above, and at the same time shew that the notochord has at this age
begun to appear. The sections marked 13-17 pass respectively through the
lines with corresponding numbers in fig. 18. Section 1 (fig. 13) passes
through the middle of the medullary canal.
In it the following points are to be noted. (1) That the epiblast becomes
very much thinner where it lines the medullary canal (_mc_), a feature
never found in the epiblast lining the primitive groove. (2) That the
mesoblast is very much thickened to form the medullary folds at _A_, _A_,
while there is no adherence between it and the epiblast, below the
primitive groove. (3) The notochord (_ch_) has begun to be formed, though
its separation from the rest of the mesoblast is not as yet very
distinct[8].
Footnote 8: In the figure the notochord has been made too
distinct.
In fig. 14 the medullary groove has become wider and the medullary folds
broader, the notochord has also become more expanded: the other features
are the same as in section 1. In the third section (fig. 15) the notochord
is still more expanded; the bottom of the now much expanded medullary
groove has become raised to form the ridge which separates the medullary
from the primitive groove. The medullary folds are also flatter and broader
than in the previous section. Section 4 (fig. 16) passes through the
anterior end of the primitive groove. Here the notochord is no longer
visible, and the adherence between the mesoblast and epiblast below the
primitive groove comes out in marked contrast with the entire separation of
the two layers in the previous sections.
The medullary folds (_A_) are still visible outside the raised edges of the
primitive groove, and are as distinctly as possible separate and
independent formations, having no connection with the folds of the
primitive groove. In the last section (fig. 17), which is taken some way
behind section 4, no trace of the medullary folds is any longer to be seen,
and the primitive groove has become deeper. This series of sections, taken
in conjunction with the specimen figured in fig. 18, must remove all
possible doubt as to the total and entire independence of the primitive and
medullary grooves. They arise in different parts of the blastoderm; the one
reaches its maximum growth before the other has commenced to be formed; and
finally, they are distinguished by almost every possible feature by which
two such grooves could be distinguished.
Soon after the formation of the notochord, the protovertebræ begin to be
formed along the sides of the medullary groove (Pl. 1, fig. 19, _pv_). Each
new protovertebra (of those which are formed from before backwards) arises
just in front of the anterior end of the primitive groove. As growth
continues, the primitive groove becomes pushed further and further back,
and becomes less and less conspicuous, till at about thirty-six hours only
a very small and curved remnant is to be seen behind the sinus
rhomboidalis; but even up to the forty-ninth Dursy has been able to
distinguish it at the hinder end of the embryo.
The primitive groove in the chick is, then, a structure which appears very
early, and soon disappears without entering directly into the formation of
any part of the future animal, and without, so far as I can see, any
function whatever. It is clear, therefore, that the primitive groove must
be the rudiment of some ancestral feature; but whether it is a rudiment of
some structure which is to be found in reptiles, or whether of some earlier
form, I am unable to decide. It is just possible that it is the last trace
of that involution of the epiblast by which the hypoblast is formed in most
of the lower animals. The fact that it is formed in the hinder part of the
pellucid area perhaps tells slightly in favour of this hypothesis, since
the point of involution of the epiblast not unfrequently corresponds with
the position of the anus.
EXPLANATION OF PLATE 1, Figs. 6-8 and 13-19.
Figs. 6 and 7 are sections through an embryo rather earlier than the one
drawn in fig. 8. Fig. 6 passes through the just commencing medullary groove
(_md_), which appears in fresh specimens, as in fig. 8, merely as an opaque
streak coming from the end of the primitive groove. The notochord is hardly
differentiated, but the _complete_ separation of mesoblast and hypoblast
under the primitive groove is clearly shewn. Fig. 7 passes through the
anterior end of the primitive groove (_pr_), and shews the fusion between
the mesoblast and epiblast, which is always to be found under the primitive
groove.
Fig. 8 is a view from above of a twenty hours' blastoderm, seen as a
transparent object. Primitive groove (_pr_). Medullary groove (_md_), which
passes off from the anterior end of the primitive groove, and is produced
by the thickening of the mesoblast. Head fold (_pf_).
Figs. 13-17 are sections through the blastoderm, drawn in fig. 18 through
the lines 1, 2, 3, 4, 5 respectively.
The first section (fig. 13) passes through the true medullary groove
(_mc_); the two medullary folds (_A_, _A_) are seen on each side with the
thickened mesoblast, and the mesoblast cells are beginning to form the
notochord (_nc_) under the medullary groove. There is no adherence between
the mesoblast cells and the epiblast under the medullary groove.
The second (fig. 14) section passes through the medullary groove where it
has become wider. Medullary folds, _A_, _A_; notochord, _ch_.
In the third section (fig. 15) the notochord (_ch_) is broader, and the
epiblast is raised in the centre, while the medullary folds are seen far
apart at _A_.
In section fig. 16 the medullary folds (_A_) are still to be seen enclosing
the anterior end of the primitive groove (_pr_). Where the primitive groove
appears there is a fusion of the epiblast and mesoblast, and no appearance
of the notochord.
In the last section, fig. 17, no trace is to be seen of the medullary
folds.
Figs. 18 and 19 are magnified views of two hardened blastoderms. Fig. 18 is
twenty-three hours old; fig. 19 twenty-five hours. They both shew how the
medullary canal arises entirely independently of the primitive groove and
in front of it, and also how the primitive groove gets pushed backwards by
the growth of the medullary groove. _pv_, Protovertebræ; other references
as above. Fig. 18 is the blastoderm from which sections figs. 13-17 were
cut.
IV. THE DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK[9].
Footnote 9: From the _Quarterly Journal of Microscopical
Science_, Vol. XIII, 1873.
With Plate 2.
The development of the first blood-vessels of the yolk-sac of the chick has
been investigated by a large number of observers, but with very discordant
results. A good historical _résumé_ of the subject will be found in a paper
of Dr Klein (liii. Band der _K. Akad. der Wissensch. Wien_), its last
investigator.
The subject is an important one in reference to the homologies of the
blood-vascular system of the vertebrata. As I shall shew in the sequel (and
on this point my observations agree with those of Dr Klein), the
blood-vessels of the chick do not arise as spaces or channels between the
cells of the mesoblast; on the contrary, they arise as a network formed by
the united processes of mesoblast-cells, and it is through these processes,
and not in the spaces between them, that the blood flows. It is, perhaps,
doubtful whether a system of vessels arising in this way can be considered
homologous with any vascular system which takes its origin from channels
hollowed out in between the cells of the mesoblast.
My own researches chiefly refer to the development of the blood-vessels in
the pellucid area. I have worked but very slightly at their development in
the vascular area; but, as far as my observations go, they tend to prove
that the mode of their origin is the same, both for the pellucid and the
vascular area.
The method which I have principally pursued has been to examine the
blastoderm from the under surface. It is very difficult to obtain exact
notions of the mode of development of the blood-vessels by means of
sections, though these come in as a valuable confirmation of the other
method.
For the purpose of examination I have employed (1) fresh specimens; (2)
specimens treated with spirit, and then mounted in glycerine; (3) specimens
treated with chloride of gold for about half a minute, and then mounted in
glycerine; and (4) specimens treated with osmic acid.
All these methods bring out the same appearances with varying clearness;
but the successful preparations made by means of the gold chloride are the
best, and bring out the appearances with the greatest distinctness.
The first traces of the blood-vessels which I have been able to distinguish
in the pellucid area are to be seen at about the thirtieth hour or slightly
earlier, at about the time when there are four to five protovertebræ on
each side.
Fig. 1 shews the appearance at this time. Immediately above the hypoblast
there are certain cells whose protoplasm sends out numerous processes.
These processes vary considerably in thickness and size, and quickly come
in contact with similar processes from other cells, and unite with them.
I have convinced myself, by the use of the hot stage, that these processes
continually undergo alteration, sometimes uniting with other processes,
sometimes becoming either more elongated and narrower or broader and
shorter. In this way a network of somewhat granular protoplasm is formed
with nuclei at the points from which the processes start.
From the first a difference may be observed in the character of this
network in different parts of the pellucid area. In the anterior part the
processes are less numerous and thicker, the nuclei fewer, and the meshes
larger; while in the posterior part the processes are generally very
numerous, and at first thin, the meshes small, and the nuclei more
frequent. As soon as this network commences to be formed the nuclei begin
to divide. I have watched this take place with the hot stage. It begins by
the elongation of the nucleus and division of the nucleolus, the parts of
which soon come to occupy the two ends of the nucleus. The nucleus becomes
still longer and then narrows in the centre and divides. By this means the
nuclei become much more numerous, and are found in almost all the larger
processes. Whether they are carried out into the processes by the movement
of the surrounding protoplasm, or whether they move through the protoplasm,
I have been unable to determine; the former view, however, seems to be the
most probable.
It is possible that some nuclei arise spontaneously in the protoplasm, but
I am much more inclined to think that they are all formed by the division
of pre-existing nuclei--a view favoured by the number of nuclei which are
seen to possess two nucleoli. Coincidently with the formation of the new
nuclei the protoplasm of the processes, as well as that surrounding the
nuclei at the starting-points of the processes, begins to increase in
quantity.
At these points the nuclei also increase more rapidly than elsewhere, but
at first the resulting nuclei seem to be all of the same kind.
In the anterior part of the pellucid area (fig. 4) the increase in the
number of nuclei and in the amount of protoplasm at the starting-points of
the protoplasm is not very great, but in the posterior part the increase in
the amount of the protoplasm at these points is very marked, and
coincidently the increase in number of the nuclei is also great. This is
shewn in figs. 2 and 3. These are both taken from the tail end of an embryo
of about thirty-three hours, with seven or eight protovertebræ. Fig. 3
shews the processes beginning to increase in thickness, and also the
protoplasm at the starting-points increasing in quantity; at the same time
the nuclei at these points are beginning to become more numerous. Fig. 3 is
taken from a slightly higher level, _i.e._ slightly nearer the epiblast. In
it the protoplasm is seen to have increased still more in quantity, and to
be filled with nuclei. These nuclei have begun to be slightly coloured, and
one of them is seen to possess two nucleoli.
Very soon after this a change in the nuclei begins to be observed, more
especially in the hinder part of the embryo. While before this time they
were generally elongated, some of them now become more nearly circular. In
addition to this, they begin to have a yellowish tinge, and the nuclei,
when treated with gold (for in the fresh condition it is not easy to see
them distinctly), have a more jagged and irregular appearance than the
nucleoli of the other nuclei.
This change takes place especially at the starting-points of the processes,
so that the appearance presented (fig. 5) is that of spherical masses of
yellowish nuclei connected with other similar spherical masses by
protoplasmic processes, in which nuclei of the original type are seen
imbedded. These masses are surrounded by a thin layer of protoplasm, at the
edge of which a normal nucleus may here and there be detected, as at
fig. 5, _a_ and _a´_, the latter possessing two nucleoli. Some of these
processes are still very delicate, and it is exceedingly probable that they
undergo further changes of position before the final capillary system is
formed.
These differentiated nuclei are the first stage in the formation of the
blood-corpuscles. From their mode of formation it is clear that the
blood-corpuscles of the Sauropsida are to be looked upon as nuclei
containing nucleoli, rather than as cells containing nuclei; indeed, they
seem to be merely ordinary nuclei with red colouring matter.
This would make them truly instead of only functionally homologous with the
red corpuscles of the Mammalia, and would well agree with the fact that the
red corpuscles of Mammalia, in their embryonic condition, possess what have
previously been called nuclei, but which might perhaps more properly be
called nucleoli.
In the anterior part of the blastoderm the processes, as I have stated, are
longer and thinner, and the spaces enclosed between them are larger. This
is clearly brought out in Pl. 2, fig. 4. But, besides these large spaces,
there are other smaller spaces, such as that at _v_. It is, on account of
the transparency of the protoplasm, very difficult to decide whether these
are vacuoles or simply spaces enclosed by the processes, but I am inclined
to think that they are merely spaces. The difficulty of exactly determining
this point is increased by the presence of numerous white-yolk spherules in
the hypoblast above, which considerably obscure the view. At about the same
time that the blood-corpuscles appear in the posterior end of the pellucid
area, or frequently a little later, they begin to be formed in the anterior
part also. The masses of them are, however, far smaller and far fewer than
in the posterior part of the embryo. It is at the tail end of the pellucid
area that the chief formation of blood-corpuscles takes place.
The part of the pellucid area intermediate in position between the anterior
and posterior ends of the embryo is likewise intermediate as regards the
number of corpuscles formed and the size of the spaces between the
processes; the spaces being here larger than at the posterior extremity,
but smaller than the spaces in front. Close to the sides of the embryo the
spaces are, however, smaller than in any other part of the pellucid area.
It is, however, in this part that the first formation of blood-corpuscles
takes place, and that the first complete capillaries are formed.
We have then somewhat round protoplasmic masses filled with
blood-corpuscles and connected by means of processes, a few of which may
begin to contain blood-corpuscles, but the majority of which only contain
ordinary nuclei. The next changes to be noticed take place in the nuclei
which were not converted into blood-corpuscles, but which were to be seen
in the protoplasm surrounding the corpuscles. They become more numerous and
smaller, and, uniting with the protoplasm in which they were imbedded,
become converted into flat cells (spindle-shaped in section), and in a
short time form an entire investment for the masses of blood-corpuscles.
The same change also occurs in the protoplasmic processes which connect the
masses of corpuscles. In the case of those processes which contain no
corpuscles the greater part of their protoplasm seems to be converted into
the protoplasm of the spindle-shaped cells. The nuclei arrange themselves
so as completely to surround the exterior of the protoplasmic processes. In
this way each process becomes converted into a hollow tube, completely
closed in by cells formed from the investment of the original nuclei by the
protoplasm which previously formed the solid processes. The remainder of
the protoplasm probably becomes fluid, and afterwards forms the plasma in
which the corpuscles float. While these changes are taking place the
formation of the blood-corpuscles does not stand still, and by the time a
system of vessels, enclosed by cellular walls, is formed out of the
protoplasmic network, a large number of the connecting processes in this
network have become filled with blood-corpuscles. The appearances presented
by the network at a slightly later stage than this is shewn in Pl. 2,
fig. 6, but in this figure all the processes are seen to be filled with
blood-corpuscles.
This investment of the masses of corpuscles by a cellular wall occurs much
earlier in some specimens than in others, both in relation to the time of
incubation and to the completion of the network. It is generally completed
in some parts by the time there are eight or nine protovertebræ, and is
almost always formed over a great part of the pellucid area by the
thirty-sixth hour. The formation of the corpuscles, as was pointed out
above, occurs earliest in the central part of the hour-glass shaped
pellucid area, and latest in its anterior part. In the hinder part of the
pellucid area the processes, as well as their enlarged starting-points,
become entirely filled with corpuscles; this, however, is by no means the
case in its anterior part. Here, although the corpuscles are undoubtedly
developed in parts as shewn in fig. 7, yet a large number of the processes
are entirely without them. Their development, moreover, is in many cases
very much later. When the development has reached the stage described, very
little is required to complete the capillary system. There are always, of
course, a certain number of the processes which end blindly, and others are
late in their development, and are not by this time opened; but, as a
general rule, when the cellular investment is formed for the masses of
corpuscles, there is completed an open network of tubes with cellular
walls, which are more or less filled with corpuscles. These become quickly
driven into the opaque area in which at that time more corpuscles may
almost always be seen than in the pellucid area.
By the formation of a network of this kind it is clear that there must
result spaces enclosed between the walls of the capillaries; these spaces
have under the microscope somewhat the appearance of being vesicles
enclosed by walls formed of spindle-shaped cells. In reality they are only
spaces enclosed at the sides, and, as a general rule, not above and below.
They have been mistaken by some observers for vesicles in which the
corpuscles were supposed to be developed, and to escape by the rupture of
the walls into the capillary spaces between. This mistake has been clearly
pointed out by Klein (_loc. cit._).
At the time when these spaces are formed, and especially in the hinder
two-thirds of the pellucid area, and in the layer of blood-vessels
immediately above the hypoblast, a formation takes place which forms in
appearance a secondary investment of the capillaries. Dr Klein was the
first to give a correct account of this formation. It results from the
cells of the mesoblast in the meshes of the capillary system. Certain of
these cells become flattened, and send out fine protoplasmic processes.
They arrange themselves so as completely to enclose the spaces between the
capillaries, forming in this way vesicles.
Where seen on section (vide fig. 6) at the edge of the vesicles these cells
lining the vesicles appear spindle-shaped, and look like a secondary
investment of the capillaries. This investment is most noticeable in the
hinder two-thirds of the pellucid area; but, though less conspicuous, there
is a similar formation in its anterior third, where there would seem to be
only veins present. Dr Klein (_loc. cit._, fig. 12) has also drawn this
investment in the anterior third of the pellucid area. He has stated that
the vessels in the mesoblast between the splanchnopleure and the
somatopleure, and which are enclosed by prolongations from the former, do
not possess this secondary investment; he has also stated that the same is
true for the sinus terminalis; but I am rather doubtful whether the
generalisation will hold, that veins and arteries can from the first be
distinguished by the latter possessing this investment. I am also rather
doubtful whether the spaces enclosed by the protoplasmic threads between
the splanchnopleure and somatopleure are the centres of vessels at all,
since I have never seen any blood-corpuscles in them.
It is not easy to learn from sections much about the first stages in the
formation of the capillaries, and it is impossible to distinguish between a
completely-formed vessel and a mere spherical space. The fine protoplasmic
processes which connect the masses of corpuscles can rarely be seen in
sections, except when they pass vertically, as they do occasionally (vide
Pl. 2, fig. 9) in the opaque area, joining the somatopleure and the
splanchnopleure. Dr Klein considers these latter processes to be the walls
of the vessels, but they appear rather to be the processes which will
eventually become new capillaries.
From sections, however, it is easy to see that the appearances of the
capillaries in the vascular area are similar to the appearances in the
pellucid area, from which it is fair to conclude that their mode of
formation is the same in both. It is also easy to see that the first
formation of vessels occurs in the splanchnopleure, and that even up to the
forty-fifth hour but few or no vessels are found in the somatopleure. The
mesoblast of the somatopleure is continued into the opaque area as a single
layer of spindle-shaped cells.
Sections clearly shew in the case of most of the vessels that the secondary
investment of Klein is present, even in the case of those vessels which lie
immediately under the somatopleure.
In reference to the origin of particular vessels I have not much to say. Dr
Klein's account of the origin of the sinus terminalis is quite correct. It
arises by a number of the masses of blood-corpuscles, similar to those
described above, becoming connected together by protoplasmic processes. The
whole is subsequently converted into a continuous vessel in the usual way.
From the first the sinus terminalis possesses cellular walls, as is clear
from its mode of origin. I am inclined to think that Klein is right in
saying that the aortæ arise in a similar manner, but I have not worked out
their mode of origin very fully.
It will be seen from the account given above that, in reference to the
first stages in the development of the blood-vessels, my observations
differ very considerably from those of Dr Klein; as to the later stages,
however, we are in tolerable agreement. We are in agreement, moreover, as
to the fundamental fact that the blood-vessels are formed by a number of
cells becoming connected, and by a series of changes converted into a
network of vessels, and that they are not in the first instance merely
channels between the cells of the mesoblast.
By the forty-fifth hour colourless corpuscles are to be found in the blood
whose exact origin I could not determine; probably they come from the walls
of the capillaries.
In the vessels themselves the coloured corpuscles undergo increase by
division, as has already been shewn by Remak. Corpuscles in the various
stages of division may easily be found. They do not appear to show very
active amoeboid movements in the vessels, though their movements are
sometimes very active when removed from the body.
To recapitulate--some of the cells of the mesoblast of the splanchnopleure
send out processes, these processes unite with the processes from other
cells, and in this way a network is formed. The nuclei of the original
cells divide, and at the points from which the processes start their
division is especially rapid. Some of them acquire especially at these
points a red colour, and so become converted into blood-corpuscles; the
others, together with part of the protoplasm in which they are imbedded,
become converted into an endothelium both for the processes and the masses
of corpuscles; the remaining protoplasm becomes fluid, and thus the
original network of the cells becomes converted into a network of hollow
vessels, filled with fluid, in which corpuscles float.
In reference to the development of the heart, my observations are not quite
complete. It is, however, easy to prove from sections (vide figs. 10 and
11, Pl. 2) that the cavity of the heart is produced by a splitting or
absorption of central cells of the thickened mesoblast of the
splanchnopleure, while its muscular walls are formed from the remaining
cells of this thickened portion. It is produced in the following way:--When
the hypoblast is folded in to form the alimentary canal the mesoblast of
the splanchnopleure follows it closely, and where the splanchnopleure turns
round to assume its normal direction (fig. 11) its mesoblast becomes
thickened. This thickened mass of mesoblast is, as can easily be seen from
figs. 10 and 11, Pl. 2, entirely distinct from the mesoblast which forms
the outside walls of the alimentary canal. At the point where this
thickening occurs an absorption takes place to form the cavity of the
heart. The method in which the cavity is formed can easily be seen from
figs. 10 and 11. It is in fig. 11 shewn as it takes place in the mesoblast
on each side, the folds of the splanchnopleure not having united in the
middle line; and hence a pair of cavities are formed, one on each side. It
is, however, probable that, in the very first formation of the heart, the
cavity is single, being formed after the two ends of the folded mesoblast
have united (vide _hz_, fig. 10). In some cases the two folds of the
mesoblast appear not at first to become completely joined in the middle
line; in this case the cavity of the heart is still complete from side to
side, but the mesoblast-cells which form its muscular walls are deficient
above. By the process of absorption, as I said, a cavity is produced in the
thickened part of the mesoblast of the splanchnopleure, a cavity which is
single in front, but becomes divided further behind, where the folds of the
mesoblast have not united, into two cavities, to form the origin of the
omphalomeseraic veins. As the folding proceeds backwards the starting-point
of the omphalomeseraic veins is also pushed backwards, and the cavities
which were before separated become joined together. From its first
formation the heart is lined internally by an endothelium; this is formed
of flattened cells, spindle-shaped in section. The exact manner of the
origin of this lining I have not been able to determine; it is, however,
probable that some of the central mesoblast-cells are directly converted
into the cells of the endothelium.
I have obtained no evidence enabling me to determine whether Dr Klein is
correct in stating that the cells of the mesoblast in the interior of the
heart become converted partly into blood-corpuscles and partly into a
cellular lining forming the endothelium of the heart, in the same way that
the blood-vessels in the rest of the blastoderm are formed. But I should be
inclined to think that it is very probable--certainly more probable than
that the cavity of the heart is formed by a process of splitting taking
place. Where I have used the word "absorption" in speaking of the formation
of the cavity of the heart, I must be understood as implying that certain
of the interior cells become converted into the endothelium, while others
either form the plasma or become blood-corpuscles.
The originally double formation of the hinder part of the heart probably
explains Dr Afanassiev's statement (_Bullétin de l'Académ. Impériale de St
Petersb._, tom. xiii, pp. 321-335), that he finds the endothelium of the
heart originally dividing its interior into two halves; for when the
partition of the mesoblast which separated at first the two halves of the
heart became absorbed, the endothelium lining of each of the originally
separate vessels would remain complete, dividing the cavity of the heart
into two parts. The partition in the central line is, however, soon
absorbed.
The account given above chiefly differs from that of Remak by not supposing
that the mesoblast-cells which form the heart are in any way split off from
the wall of the alimentary canal.
There can be no doubt that His is wrong in supposing that the heart
originates from the mesoblast of the splanchnopleure and somatopleure
uniting to form its walls, thus leaving a cavity between them in the
centre. The heart is undoubtedly formed out of the mesoblast of the
splanchnopleure only.
Afanassiev's observations are nearer to the truth, but there are some
points in which he has misinterpreted his sections.
Sections Pl. 2, figs. 10 and 11, explain what I have just said about the
origin of the heart. Immediately around the notochord the mesoblast is not
split, but a very little way outside it is seen to be split into two parts
_so_ and _sp_; the former of these follows the epiblast, and together with
it forms the somatopleure, which has hardly begun to be folded at the line
where the sections are taken. The latter (_sp_) forms with the hypoblast
(_hy_) the splanchnopleure, and thus has become folded in to form the walls
of the alimentary canal (_d_). In fig. 11 the folds have not united in the
central line, but in fig. 10 they have so united. In fig. 11, where the
mesoblast, still following the hypoblast, turns back to assume its normal
direction, it is seen to be thickened and to have become split, so that a
cavity (_of_) (of the omphalomeseraic vein) is formed in it on each side,
lined by endothelium.
In the section immediately behind section fig. 11 the mesoblast was
thickened, but had not become split.
In fig. 10 the hypoblast folds are seen to have united in the centre, so as
to form a completely closed digestive canal (_d_); the folds of the
mesoblast have also united, so that there is only a single cavity in the
heart (_hz_), lined, as was the case with the omphalomeseraic veins, by
endothelium.
In conclusion, I have to thank Dr Foster for his assistance and suggestions
throughout the investigations which have formed the subject of these three
short papers, and which were well carried on in the apartments used by him
as a Physiological Laboratory.
EXPLANATION OF PLATE 2.
Fig. 1 is taken from the anterior part of the pellucid area of a thirty
hours' chick, with four protovertebræ. At _n_ is a nucleus with two
nucleoli.
Figs. 2 and 3 are taken from the posterior end of the pellucid area of a
chick with eight protovertebræ. In fig. 3 the nuclei are seen to have
considerably increased in number at the points of starting of the
protoplasmic processes. At _n_ is seen a nucleus with two nucleoli.
Fig. 4 is taken from the anterior part of the pellucid area of an embryo of
thirty-six hours. It shews the narrow processes characteristic of the
anterior part of the pellucid area, and the fewer nuclei. Small spaces,
which have the appearance of vacuoles, are shewn at _v_.
Fig. 5 is taken from the posterior part of the pellucid area of a
thirty-six hours' embryo. It shews the nuclei, with somewhat irregular
nucleoli, which have begun to acquire the red colour of blood-corpuscles;
the protoplasmic processes containing the nuclei; the nuclei in the
protoplasm surrounding the corpuscles, as shewn at _a_, _a´_.
Fig. 6 shews fully formed blood-vessels, in part filled with
blood-corpuscles and in part empty. The walls of the capillaries, formed of
cells, spindle-shaped in section, are shewn, and also the secondary
investment of Klein at _k_, and at _b_ is seen a narrow protoplasmic
process filled with blood-corpuscles.
Fig. 7 is taken from the anterior part of the pellucid area of a thirty-six
hours' embryo. It shews a collection of nuclei which are beginning to
become blood-corpuscles.
Figs. 1-5 are drawn with an 1/8 object-glass. Fig. 6 is on a much smaller
scale. Fig. 7 is intermediate.
Fig. 8.--A transverse section through the dorsal region of a forty-five
hours' embryo; _ao_, aorta with a few blood-corpuscles. v, Blood-vessels,
all of them being formed in the splanchnopleure, and all of them provided
with the secondary investment of Klein; _pe_, pellucid area; _op_, opaque
area.
Fig. 9.--Small portion of a section through the opaque area of a
thirty-five hours' embryo, showing protoplasmic processes, with nuclei
passing from the somatopleure to the splanchnopleure.
Fig. 10.--Section through the heart of a thirty-four hours' embryo. _a_.
Alimentary canal; _hb_, hind brain; _nc_, notochord; _e_, epiblast; _so_,
mesoblast of the somatopleure; _sp_, mesoblast of the splanchnopleure;
_hy_, hypoblast; _hz_, cavity of the heart.
Fig. 11.--Section through the same embryo as fig. 10, and passing through
the orifice of the omphalomeseraic vein. _of_, Omphalomeseraic vein; other
references as above.
These two sections shew that the heart is entirely formed from the
mesoblast of the _splanchnopleure_, and that it is formed by the splitting
of that part of the mesoblast which has turned to assume its normal
direction after being folded in to form the muscular wall of the alimentary
canal. In fig. 11 the cavities so formed on each side have not yet united,
but in fig. 10 they have united. When the folding becomes more complete the
cavities (_of_, _of_) in fig. 11 will unite, and in this way the origin of
the omphalomeseraic veins will be carried further backwards. In the section
immediately behind section 11 the mesoblast had become thickened, but had
not split.
V. A PRELIMINARY ACCOUNT OF THE DEVELOPMENT OF THE ELASMOBRANCH FISHES[10].
Footnote 10: From the _Quarterly Journal of Microscopical
Science_, Vol. XIV. 1874. Read in Section D, at the Meeting of
the British Association at Belfast.
With Plates 3 and 4.
During the spring of the present year I was studying at the Zoological
Station, founded by Dr Dohrn at Naples, and entirely through its agency was
supplied with several hundred eggs of various species of Dog-fish
(Selachii)--a far larger number than any naturalist has previously had an
opportunity of studying. The majority of the eggs belonged to an oviparous
species of _Mustelus_, but in addition to these I had a considerable number
of eggs of two or three species of _Scyllium_, and some of the Torpedo.
Moreover, since my return to England, Professor Huxley has most liberally
given me several embryos of _Scyllium stellare_ in a more advanced
condition than I ever had at Naples, which have enabled me to fill up some
lacunæ in my observations.
On many points my investigations are not yet finished, but I have already
made out a number of facts which I venture to believe will add to our
knowledge of vertebrate embryology; and since it is probable that some time
will elapse before I am able to give a complete account of my
investigations, I have thought it worth while preparing a preliminary paper
in which I have briefly, but I hope in an intelligible manner, described
some of the more interesting points in the development of the
Elasmobranchii. The first-named species (_Mustelus_ sp.?) was alone used
for the early stages, for the later ones I have also employed the other
species, whose eggs I have had; but as far as I have seen at present, the
differences between the various species in early embryonic life are of no
importance.
Without further preface I will pass on to my investigations.
_The Egg-shell._
In the eggs of all the species of Dog-fishes which I have examined the yolk
lies nearest that end of the quadrilateral shell which has the shortest
pair of strings for attachment. This is probably due to the shape of the
cavity of the shell, and is certainly not due to the presence of any
structures similar to chalazæ.
_The Yolk._
The yolk is not enclosed in any membrane comparable to the vitelline
membrane of Birds, but lies freely in a viscid albumen which fills up the
egg-capsule. It possesses considerable consistency, so that it can be
removed into a basin, in spite of the absence of a vitelline membrane,
without falling to pieces. This consistency is not merely a property of the
yolk-sphere as a whole, but is shared by every individual part of it.
With the exception of some finely granular matter around the blastoderm,
the yolk consists of rather small, elliptical, highly refracting bodies,
whose shape is very characteristic and renders them easily recognizable. A
number of striæ like those of muscle are generally visible on most of the
spherules, which give them the appearance of being in the act of breaking
up into a series of discs; but whether these striæ are normal, or produced
by the action of water I have not determined.
_Position of the Blastoderm._
The blastoderm is always situated, immediately after impregnation, near the
pole of the yolk which lies close to the end of the egg-capsule. Its
position varies a little in the different species and is not quite constant
in different eggs of the same species. But this general situation is quite
invariable. It is of about the same proportional size as the blastoderm of
a bird.
_Segmentation._
In a fresh specimen, in which segmentation has only just commenced, the
blastoderm or germinal disc appears as a circular disc, distinctly marked
off by a dark line from the rest of the yolk. This line, as is proved by
sections, is the indication of a very shallow groove. The appearance of
sharpness of distinction between the germ and the yolk is further
intensified by their marked difference of colour, the germ itself being
usually of a darker shade than the remainder of the yolk; while around its
edge, and apparently sharply separated from it by the groove before
mentioned, is a ring of a different shade which graduates at its outer
border into the normal shade of the yolk.
These appearances are proved by transverse sections to be deceptive. There
is no sharp line either at the sides or below separating the blastoderm
from the yolk. In the passage between the fine granular matter of the germ
to the coarser yolk-spheres every intermediate size of granule is present;
and, though the space between the two is rather narrow, in no sense of the
word can there be said to be any break or line between them.
This gradual passage stands in marked contrast with what we shall find to
be the case at the close of the segmentation. In the youngest egg which I
had, the germinal disc was already divided into four segments by two
furrows at right angles. These furrows, however, did not reach its edge;
and from my sections I have found that they were not cut off below by any
horizontal furrow. So that the four segments were continuous below with the
remainder of the germ without a break.
In the next youngest specimen which I had, there were already present
eighteen segments, somewhat irregular in size, but which might roughly be
divided into an outer ring of larger spheres, separated, as it were, by a
circular furrow from an inner series of smaller segments. The furrows in
this case reached quite to the edge of the germinal disc.
The remarks I made in reference to the earlier specimen about the
separation of the germ from the yolk apply in every particular to the
present one. The external limit of the blastoderm was not defined by a true
furrow, and the segmentation furrows still ended below without meeting any
horizontal furrows, so that the blastoderm was not yet separated by any
line from the remainder of the yolk, and the segments of which it was
composed were still only circumscribed upon five sides. In this particular
the segmentation in these animals differs materially from that in the Bird,
where the horizontal furrows appear very early.
In each segment a nucleus was generally to be seen in sections. I will,
however, reserve my remarks upon the nature of the nuclei till I discuss
the nuclei of the blastoderm as a whole.
For some little time the peripheral segments continue larger than the more
central ones, but this difference of size becomes less and less marked, and
before the segments have become too small to be seen with the simple
microscope, their size appears to be uniform over the whole surface of the
blastoderm.
In the blastoderms somewhat older than the one last described the segments
have already become completely separate masses, and each of them already
possesses a distinct nucleus. They form a layer one or two segments deep.
The limits of the blastoderm are not, however, defined by the already
completed segments, but outside these new segments continue to be formed
around nuclei which appear in the yolk. At this stage there is, therefore,
no line of demarcation between the germ and the yolk, but the yolk is being
bored into, so to speak, by a continuous process of fresh segmentation.
The further segmentation of the already existing spheres, and the formation
of new ones from the yolk below and to the sides, continues till the
central cells acquire their final size, the peripheral ones being still
large, and undefined towards the yolk. These also soon reach the final
size, and the blastoderm then becomes rounded off towards the yolk and
sharply separated from it.
_The Nuclei of the Yolk._
Intimately connected with the segmentation is the appearance and history of
a number of nuclei which arise in the yolk surrounding the blastoderm.
When the horizontal furrows appear which first separate the blastoderm from
the yolk, the separation does not occur along the line of passage from the
fine to the coarse yolk, but in the former at some distance from this line.
The blastoderm thus rests upon a mass of finely granular material, from
which, however, it is sharply separated. At this time there appear in this
finely granular material a number of nuclei of a rather peculiar character.
They vary immensely in size--from that of an ordinary nucleus to a size
greater than the largest blastoderm-cell.
In Pl. 3, fig. 1, _n_, is shewn their distribution in this finely granular
matter and their variation in size. But whatever may be their size, they
always possess the same characteristic structure. This is shewn in Pl. 3,
figs. 1 and 2, _n_.
They are rather irregular in shape, with a tendency when small to be
roundish, and are divided by a number of lines into distinct areas, in each
of which a nucleolus is to be seen. The lines dividing them into these
areas have a tendency (in the smaller specimens) to radiate from the
centre, as shewn in Pl. 3, fig. 1.
These nuclei colour red with hematoxylin and carmine and brown with osmic
acid, while the nucleoli or granules contained in the areas also colour
_very intensely_ with all the three above-named reagents.
With such a peculiar structure, in favourable specimens these nuclei are
very easily recognised, and their distribution can be determined without
difficulty. They are not present alone in the finely granular yolk, but
also in the coarsely granular yolk adjoining it. They form very often a
special row, sometimes still more markedly than in Pl. 3, fig. 1, along the
floor of the segmentation cavity. They are not, however, found alone in the
yolk. All the blastoderm-cells in the earlier stages possess precisely
similar nuclei! From the appearance of the first nucleus in a
segmentation-sphere till a comparatively late period in development, every
nucleus which can be distinctly seen is found to be of this character. In
Pl. 3, fig. 2, this is very distinctly shewn.
(1) We have, then, nuclei of this very peculiar character scattered through
the sub-germinal granular matter, and also universally present in the cells
of the blastoderm. (2) These nuclei are distributed in a special manner
under the floor of the segmentation cavity on which new cells are
continually appearing. Putting these two facts together, there would be the
strongest presumption that these nuclei do actually become the nuclei of
cells which enter the blastoderm, and such is actually the case. In my
account of the segmentation I have, indeed, already mentioned this, and I
will return to it, but before doing so will enter more fully into the
distribution of these nuclei in the yolk.
They appear in small numbers around the blastoderm at the close of
segmentation, and round each one of them there may at this time be seen in
osmic acid specimens, and with high powers, a fine network similar to but
finer than that represented in Pl. 3, fig. 2. This network cannot, as a
general rule, be traced far into the yolk, but in some exceptionally thin
specimens it may be seen in any part of the fine granular yolk around the
blastoderm, the meshes of the network being, however, considerably coarser
between than around the nuclei. This network may be seen in the fine
granular material around the germ till the latest period of which I have
yet cut sections of the blastoderm. In the later specimens, indeed, it is
very much more distinctly seen than in the earlier, owing to the fact that
in parts of the blastoderm, especially under the embryo, the yolk-granules
have disappeared partly or entirely, leaving only this fine network with
the nuclei in it.
A specimen of this kind is represented in Pl. 3, fig. 2, where the meshes
of the network are seen to be finer immediately around the nuclei, and
coarser in the intervals. The specimen further shows in the clearest manner
that this network is not divided into areas, each representing a cell and
each containing a nucleus. I do not know to what extent this network
extends into the yolk. I have never yet seen the limits of it, though it is
very common to see the coarsest yolk-granules lying in its meshes. Some of
these are shewn in Pl. 3, fig. 2, _yk_.
This network of lines[11] (probably bubbles) is characteristic of many
cells, especially ova. We are, therefore, forced to believe that the fine
granular and probably coarser granular yolk of this meroblastic egg
consists of an active organized basis with passive yolk-spheres imbedded in
it. The organized basis is especially concentrated at the germinal pole of
the egg, but becomes less and less in quantity, as compared with the
yolk-spheres, the further we depart from this.
Footnote 11: The interpretation of this network is entirely
due to Dr Kleinenberg, who suggested it to me on my shewing him
a number of specimens exhibiting the nuclei and network.
Admitting, as I think it is necessary to do, the organized condition of the
whole yolk-sphere, there are two possible views as to its nature. We may
either take the view that it is one gigantic cell, the ovum, which has
grown at the expense of the other cells of the egg-follicle, and that these
cells in becoming absorbed have completely lost their individuality; or we
may look upon the true formative yolk (as far as we can separate it from
the remainder of the food-yolk) as the remains of one cell (the primitive
ovum), and the remainder of the yolk as a body formed from the coalescence
of the other cells of the egg-follicle, which is adherent to, but has not
coalesced with, the primitive ovum, the cells in this case not having
completely lost their individuality; and to these cells, the nuclei, I have
found, must be supposed to belong.
The former view I think, for many reasons, the most probable. The share of
these nuclei in the segmentation, and the presence of similar nuclei in the
cells of the germ, both support it, and are at the same time difficulties
in the way of the other view. Leaving this question which cannot be
discussed fully in a preliminary paper like the present one, I will pass on
to another important question, viz.:
How do these nuclei originate? Are they formed by the division of the
pre-existing nuclei, or by an independent formation? It must be admitted
that many specimens are strongly in favour of the view that they increase
by division. In the first place, they are often seen "two together;"
examples of this will be seen in Pl. 3, fig. 1. In the second place, I have
found several specimens in which five or six appear close together, which
look very much as if there had been an actual division into six nuclei. It
is, however, possible in this case that the nuclei are really connected
below and only appear separate, owing to the crenate form of the mass.
Against this may be put the fact that the division of a nucleus is by no
means so common as has been sometimes supposed, that in segmentation it has
very rarely been observed that the nucleus of a sphere first divides[12],
and that then segmentation takes place, but segmentation generally occurs
and then a new nucleus arises in each of the newly formed spheres. Such
nuclei as I have described are rare; they have, however, been observed in
the egg of a _Nephelis_ (one of the Leeches), and have in that case been
said to divide. Dr Kleinenberg, however, by following a single egg through
the whole course of its development, has satisfied himself that this is not
the case, and that, further, these nuclei in Nephelis never form the nuclei
of newly developing cells.
Footnote 12: Kowalevsky ("Beiträge zur Entwicklungsgeschichte
der Holothurien," _Mémoires de l'Ac. Imp. de St Petersbourg_,
vii ser., Vol. XI. 1867) describes the division of nuclei
during segmentation in the Holothurians, and other observers
have described it elsewhere.
I must leave it an open question, and indeed one which can hardly be solved
from sections, whether these nuclei arise freely or increase by division,
but I am inclined to believe that both processes may possibly take place.
In any case their division does not appear to determine the segmentation or
segregation of the protoplasm around them.
As was mentioned in my account of the segmentation, these nuclei first
appear during that process, and become the nuclei of the freshly formed
segmentation spheres. At the close of segmentation a few of them are still
to be seen around the blastoderm, but they are not very numerous.
From this period they rapidly increase in number, up to the commencement of
the formation of the embryo as a body distinct from the germ. Though before
this period they probably become the nuclei of veritable cells which enter
the germ, it is not till this period, when the growth of the blastoderm
becomes very rapid and it commences to spread over the yolk, that these new
cells are formed in large numbers. I have many specimens of this age which
shew the formation of these new cells with great clearness. This is most
distinctly to be seen immediately below the embryo, where the
yolk-spherules are few in number. At the opposite end of the blastoderm I
believe that more of these cells are formed, but, owing to the presence of
numerous yolk-spherules, it is much more difficult to make certain of this.
As to the final destination of these cells, my observations are not yet
completed. Probably a large number of them are concerned in the formation
of the vascular system, but I will give reasons later on for believing that
some of them are concerned in the formation of the walls of the digestive
canal and of other parts.
I will conclude my account of these nuclei by briefly summarizing the
points I have arrived at in reference to them.
A portion, or more probably the whole, of the _yolk_ of the Dog-fish
consists of _organized material_, in which nuclei appear and increase
either by _division_ or by a process of _independent_ formation, and a
great number of these subsequently become the nuclei of cells formed around
them, frequently at a distance from the germ, which then travel up and
enter it.
The formation of cells in the yolk, apart from the general process of
segmentation, has been recognised by many observers. Kupffer (_Archiv. für
Micr. Anat._, Bd. IV. 1868) and Owsjannikow ("Entwicklung der Coregonus,"
_Bulletin der Akad. St Petersburgh_, Vol. XIX.) in osseous fishes[13], Ray
Lankester (_Annals and Mag. of Nat. Hist._ Vol. XI. 1873, p. 81) in
Cephalopoda, Götte (_Archiv. für Micr. Anat._ Vol. X.) in the chick, have
all described a new formation of cells from the so-called food-yolk. The
organized nature of the whole or part of this, previous to the formation of
the cells from it, has not, however, as a rule, been distinctly recognised.
In the majority of cases, as, for instance, in Loligo, the nucleus is not
the first thing to be formed, but a plastide is first formed, in which a
nucleus subsequently makes its appearance.
Footnote 13: Götte, at the end of a paper on "The Development
of the Layers in the Chick" (_Archiv. für Micr. Anat._, Vol. X.
1873, p. 196), mentions that the so-called cells in Osseous
fishes which Oellacher states to have migrated into the yolk,
and which are clearly the same as those mentioned by
Owsjannikow, are really not _cells_, but large _nuclei_. If
this statement is correct the phenomena in Osseous fishes are
precisely the same as those I have described in the Dog-fish.
_Formation of the Layers._
Leaving these nuclei, I will now pass on to the formation of the layers.
At the close of segmentation the surface of the blastoderm is composed of
cells of a uniform size, which, however, are too small to be seen by the
aid of the simple microscope.
The cells of this uppermost layer are somewhat columnar, and can be
distinguished from the remainder of the cells of the blastoderm as a
separate layer. This layer forms the epiblast; and the Dog-fish agree with
Birds, Batrachians, and Osseous fish in the very early differentiation of
it.
The remainder of the cells of the blastoderm form a mass, many cells deep,
in which it is impossible as yet or till a very considerably later period
to distinguish two layers. They may be called the _lower layer cells_. Some
of them near the edge of this mass are still considerably larger than the
rest, but they are, as a whole, of a fairly uniform size. Their nuclei are
of the same character as the nuclei in the yolk.
There is one point to be noticed in the shape of the blastoderm as a whole.
It is unsymmetrical, and a much larger number of its cells are found
collected at one end than at the other. This absence of symmetry is found
in all sections which are cut parallel to the long axis of the egg-capsule.
The thicker end is the region where the embryo will subsequently appear.
This very early appearance of distinction in the blastoderm between the end
at which the embryo will appear, and the non-embryonic end is important,
especially as it shews the affinity of the modes of development of Osseous
fishes and the Elasmobranchii. Oellacher (_Zeitschrift für Wiss. Zoologie_,
Vol. XXXIII. 1873) has shewn, and, though differing from him on many other
points, on this point Götte (_Arch. für Micr. Anat._ Vol. IX. 1873) agrees
with him, that a similar absence of symmetry by which the embryonic end of
the germ is marked off, occurs almost immediately after the end of
segmentation in Osseous fishes. In the early stages of development there
are a number of remarkable points of agreement between the Osseous fish and
the Dog-fish, combined with a number of equally remarkable points of
difference. Some of these I shall point out as I proceed with my
description.
The embryonic end of the germ is always the one which points towards the
pole of the yolk farthest removed from the egg-capsule.
The germ grows, but not very rapidly, and without otherwise undergoing any
very appreciable change, for some time.
The growth at these early periods appears to be particularly slow,
especially when compared with the rapid manner in which some of the later
stages of the development are passed through.
The next important change which occurs is the formation of the so-called
"segmentation cavity."
This forms a very marked feature throughout the early stages. It appears,
however, to have somewhat different relations to the blastoderm than the
homologous structure in other vertebrates. In its earliest stage which I
have observed, it appears as a small cavity in the centre of the lower
layer cells. This grows rapidly, and its roof becomes composed of epiblast
and only a thin lining of "_lower layer_" cells, while its floor is formed
by the yolk (Pl. 3, fig. 3, _sg_). In the next and third stage (Pl. 3,
fig. 4, _sg_) its floor is formed by a thin layer of cells, its roof
remaining as before. It has, however, become a less conspicuous formation
than it was; and in the last (fourth) stage in which it can be
distinguished it is very inconspicuous, and almost filled up by cells.
What I have called the second stage corresponds to a period in which no
trace of the embryo is to be seen. In the third stage the embryonic end of
the blastoderm projects outwards to form a structure which I shall speak of
as the "embryonic rim," and in the fourth and last stage a distinct
medullary groove is formed. For a considerable period during the second
stage the segmentation cavity remains of about the same size; during the
third stage it begins to be encroached upon, and becomes smaller both
absolutely, and relatively to the increased size of the germ.
The segmentation cavity of the Dog-fish most nearly agrees with that of
Osseous fishes in its mode of formation and relation to the embryo.
Dog-fish resemble Osseous fish in the fact that their embryos are entirely
formed from a portion of the germ which does not form part of the roof of
the segmentation cavity, so that the cells forming the roof of the
segmentation cavity take _no share_ at any time in the formation of their
embryos. They further agree with Osseous fish (always supposing that the
descriptions of Oellacher, _loc. cit._, and Götte, _Archiv. für Micr.
Anat._ Bd. IX. are correct) in the floor of the segmentation cavity being
formed at one period by yolk. Together with these points of similarity
there are some important differences.
(1) The segmentation cavity in the Osseous fish from the first arises as a
cavity between the yolk and the blastoderm, and its floor is never at any
period covered with cells. In the Dog-fish, as we have said above, both in
the earlier and later periods the floor is covered with cells.
(2) The roof in the Dog-fish is _invariably_ formed by the epiblast and a
row of flattened lower layer cells.
According to both Götte and Oellacher the roof of the segmentation cavity
in Osseous fishes is in the earlier stages formed _alone_ of the two layers
which correspond with the single layer forming the epiblast in the
Dog-fish. In Osseous fishes it is very difficult to distinguish the various
layers, owing to the similarity of their component cells. In Dog-fish this
is very easy, owing to the great distinctness of the epiblast, and it
appears to me, on this account, very probable that the two above-named
observers may be in error as to the constitution of its roof in the Osseous
fish. With both the Bird and the Frog the segmentation cavity of the
Dog-fish has some points of agreement, and some points of difference, but
it would take me too far from my present subject to discuss them.
When the segmentation cavity is first formed, no great changes have taken
place in the cells forming the blastoderm. The upper layer--the
_epiblast_--is composed of a single layer of columnar cells, and the
remainder of the cells of blastoderm, forming the lower layer, are of a
fairly uniform size, and polygonal from mutual pressure. The whole edge of
the blastoderm is thickened, but this thickening is especially marked at
its embryonic end.
This thickened edge of the blastoderm is still more conspicuous in the next
and second stage (Pl. 3, fig. 3).
In the second stage the chief points of progress, in addition to the
increased thickness of the edge of the blastoderm, are--
(1) The increased thickness and distinctness of the epiblast, caused by its
cells becoming more columnar, though it remains as a one-cell-thick layer.
(2) The disappearance of the cells from the floor of the segmentation
cavity.
The lower layer cells have undergone no important changes, and the
blastoderm has increased very little if at all in size.
From Pl. 3, fig. 3, it is seen that there is a far larger collection of
cells at the embryonic than at the opposite end.
Passing over some rather unimportant stages, I will come to the next
important one.
The general features of this (the third) stage in a surface view are--
(1) The increase in size of the blastoderm.
(2) The diminution in size of the segmentation cavity, both relatively and
absolutely.
(3) The appearance of a portion of the blastoderm projecting beyond the
rest over the yolk. This projecting rim extends for nearly half the
circumference of the yolk, but is most marked at the point where the embryo
will shortly appear. I will call it the "embryonic rim."
These points are still better seen from sections than from surface views,
and will be gathered at once from an inspection of Pl. 3, fig. 4.
The epiblast has become still more columnar, and is markedly thicker in the
region where the embryo will appear. But its most remarkable feature is
that at the outer edge of the "embryonic rim" (_er_) it turns round and
becomes continuous with the lower layer cells. This feature is most
important, and involves some peculiar modifications in the development. I
will, however, reserve a discussion of its meaning till the next stage.
The only other important feature of this stage is the appearance of a layer
of cells on the floor of the segmentation cavity.
Does this layer come from an ingrowth from the thickened edge of the
blastoderm, or does it arise from the formation of new cells in the yolk?
It is almost impossible to answer this question with certainty. The
following facts, however, make me believe that the newly formed cells do
play an important part in the formation of this layer.
(1) The presence at an earlier date of almost a row of nuclei under the
floor of the segmentation cavity (Pl. 3, fig. 1).
(2) The presence on the floor of the cavity of such large cells as those
represented in fig. 1, _bd_, cells which are very different, as far as the
size and granules are concerned, from the remainder of the cells of the
blastoderm.
On the other hand, from this as well as other sections, I have satisfied
myself that there is a distinct ingrowth of cells from the embryonic
swelling. It is therefore most probable that both these processes, viz. a
fresh formation and an ingrowth, have a share in the formation of the layer
of cells on the floor of the segmentation cavity.
In the next stage we find the embryo rising up as a distinct body from the
blastoderm, and I shall in future speak of the body, which now becomes
distinct as the embryo. It corresponds with what Kupffer (_loc. cit._) in
his paper on the "Osseous Fishes" has called the "embryonic keel." This
starting-point for speaking of the embryo as a distinct body is purely
arbitrary and one merely of convenience. If I wished to fix more correctly
upon a period which could be spoken of as marking the commencing formation
of the embryo, I should select the time when structures first appear to
mark out the portion of the germ from which the embryo becomes formed; this
period would be in the Elasmobranchii, as in the Osseous fish, at the
termination of segmentation, when the want of symmetry between the
embryonic end of the germ and the opposite end first appears.
I described in the last stage the appearance of the "embryonic rim." It is
in the middle point of this, where it projects most, that the formation of
the embryo takes place. There appear two parallel folds extending from the
edge of the blastoderm towards the centre, and cut off at their central end
by another transverse fold. These three folds raise up, between them, a
flat broadish ridge, "_the embryo_" (Pl. 3, fig. 5). The head end of the
embryo is the end nearest the centre of the blastoderm, the tail end being
the one formed by its (the blastoderm's) edge.
Almost from its first appearance this ridge acquires a shallow groove--the
medullary groove (Pl. 3, fig. 5, _mg_)--along its middle line, where the
epiblast and hypoblast are in absolute contact (vide fig. 6_a_, 7_a_, 7_b_,
&c.) and where the mesoblast (which is already formed by this stage) is
totally absent. This groove ends abruptly a little before the front end of
the embryo, and is deepest in the middle and wide and shallow behind.
On each side of it is a plate of mesoblast equivalent to the combined
vertebral and lateral plates of the Chick. These, though they cannot be
considered as entirely the cause of the medullary groove, may perhaps help
to make it deeper. In the parts of the germ outside the embryo the
mesoblast is again totally absent, or, more correctly, we might say that
outside the embryo the _lower layer cells_ do not become differentiated
into hypoblast and mesoblast, and remain continuous only with the lower of
the two layers into which the _lower layer cells_ become differentiated in
the body of embryo. This state of things is not really very different from
what we find in the Chick. Here outside the embryo (_i.e._ in the opaque
area) there is a layer of cells in which no differentiation into hypoblast
and mesoblast takes place, but the layer remains continuous rather with the
hypoblast than the mesoblast.
There is one peculiarity in the formation of the mesoblast which I wish to
call attention to, _i.e._ its formation as two lateral masses, one on each
side of the middle line, but not continuous across this line (vide figs.
6_a_ and 6_b_, and 7_a_ and 7_b_). Whether this remarkable condition is the
most primitive, _i.e._ whether, when in the stage before this the mesoblast
is first formed, it is only on each side of the middle line that the
differentiation of the lower layer cells into hypoblast and mesoblast takes
place, I do not certainly know, but it is undoubtedly a very early
condition of the mesoblast. The condition of the mesoblast as two plates,
one on each side of the neural canal, is precisely similar to its embryonic
condition in many of the Vermes, _e.g. Euaxes_ and _Lumbricus_. In these
there are two plates of mesoblast, one on each side of the nervous cord,
which are known as the _Germinal streaks_ (Keimstreifen) (vide Kowalevsky
"Würmern u. Arthropoden"; _Mém. de l'Acad. Imp. St Petersbourg_, 1871).
From longitudinal sections I have found that the segmentation cavity has
ceased by this stage to have any distinct existence, but that the whole
space between the epiblast and the yolk is filled up with a mass of
elongated cells, which probably are solely concerned in the formation of
the vascular system. The thickened posterior edge of the blastoderm is
still visible.
At the embryonic end of the blastoderm, as I pointed out in an earlier
stage, the epiblast and the lower layer cells are perfectly continuous.
Where they join the epiblast, the _lower layer cells_ become distinctly
divided, and this division commenced even in the earlier stage, into two
layers; a lower one, more directly continuous with the epiblast, consisting
of cells somewhat resembling the epiblast-cells, and an upper one of more
flattened cells (Pl. 3, fig. 4, _m_). The first of these forms the
hypoblast, and the latter the mesoblast. They are indicated by _hy_ and _m_
in the figures. The hypoblast, as I said before, remains continuous with
the whole of the rest of lower layer cells of the blastoderm (vide
fig. 7_b_). This division into hypoblast and mesoblast commences at the
earlier stage, but becomes much more marked during this one.
In describing the formation of the hypoblast and mesoblast in this way I
have assumed that they are formed out of the large mass of lower layer
cells which underlie the epiblast at the embryonic end of the blastoderm.
But there is another and, in some ways, rather a tempting view, viz. to
suppose that the epiblast, where it becomes continuous with the hypoblast,
in reality becomes involuted, and that from this involuted epiblast are
formed the whole mesoblast and hypoblast.
In this case we would be compelled to suppose that the mass of lower layer
cells which forms the embryonic swelling is used as food for the growth of
the involuted epiblast, or else employed solely in the growth over the yolk
of the non-embryonic portion of the blastoderm; but the latter possibility
does not seem compatible with my sections.
I do not believe that it is possible, from the examination of sections
alone, to decide which of these two views (viz. whether the epiblast is
involuted, or whether it becomes merely continuous with the lower layer
cells) is the true one. The question must be decided from other
considerations.
The following ones have induced me to take the view that there is no
involution, but that the mesoblast and hypoblast are formed from the lower
layer cells.
(1) That it would be rather surprising to find the mass of lower layer
cells which forms the "embryo swelling" playing no part in the formation of
embryo.
(2) That the view that it is the lower layer cells from which the hypoblast
and mesoblast are derived agrees with the mode of formation of these two
layers in the Bird, and also in the Frog; since although, in the latter
animal, there is an involution, this is not of the epiblast, but of the
larger cells of the lower pole of the yolk, which in part correspond with
what I have called the lower layer cells in the Dog-fish.
If the view be accepted that it is from the lower layer cells that the
hypoblast and mesoblast are formed, it becomes necessary to explain what
the continuity of the hypoblast with the epiblast means.
The explanation of this is, I believe, the keystone to the whole position.
The vertebrates may be divided as to their early development into two
classes, viz. those with _holoblastic ova_, in which the digestive canal is
formed by an _involution_ with the presence of an "_anus of Rusconi_."
This class includes "Amphioxus," the "Lamprey," the "Sturgeon," and
"Batrachians."
The second class are those with _meroblastic ova_ and no _anus of Rusconi_,
and with an alimentary canal formed by the infolding of the sheet of
hypoblast, the digestive canal remaining in communication with the
food-yolk for the greater part of embryonic life by an umbilical canal.
This class includes the "Elasmobranchii," "Osseous fish," "Reptiles," and
"Aves."
The mode of formation of the alimentary canal in the first class is clearly
the more primitive; and it is equally clear that its mode of formation in
the second class is an adaptation due to the presence of the large quantity
of food-yolk.
In the Dog-fish I believe that we can see, to a certain extent, how the
change from the one to the other of these modes of development of the
alimentary canal took place.
In all the members of the first class, viz. "_Amphioxus_," the "Lamprey,"
the "Sturgeon," and the "Batrachians," the epiblast becomes continuous with
the hypoblast at the so-called "anus of Rusconi," and the alimentary canal,
potentially in all and actually in the Sturgeon (vide Kowalevsky,
Owsjannikow, and Wagner, _Bulletin der Acad. d. St Petersbourg_, Vol. XIV.
1870, "Entwicklung der Störe"), communicates freely at its extreme hind end
with the neural canal. The same is the case in the Dog-fish. In these, when
the folding in to form the alimentary canal on the one hand, and the neural
on the other, takes place, the two foldings unite at the corner, where the
epiblast and hypoblast are in continuity, and place the two tubes, the
neural and alimentary, in free communication with each other[14].
Footnote 14: This has been already made out by Kowalevsky,
"Würmern u. Arthropoden," _loc. cit._
There is, however, nothing corresponding with the "anus of Rusconi," which
merely indicates the position of the involution of the digestive canal, and
subsequently completely closes up, though it nearly coincides in position
with the true anus in the Batrachians, &c.
This remarkable point of similarity between the Dog-fish's development and
the normal mode of development in the first class (the holoblastic) of
vertebrates, renders it quite clear that the continuity of the epiblast and
hypoblast in the Dogfish is really the remnant of a more primitive
condition, when the alimentary canal was formed by an involution. Besides
the continuity between neural and alimentary canals, we have other remnants
of the primitive involution. Amongst these the most marked is the formation
of the embryonic rim, which is nothing less than the commencement of an
involution. Its form is due to the flattened, sheet-like condition of the
germ. In the mode in which the alimentary canal is closed in front I shall
shew there are indications of the primitive mode of formation of the
alimentary canal; and in certain peculiarities of the anus, which I shall
speak of later, we have indications of the primitive anus of Rusconi; and
finally, in the general growth of the epiblast (small cells of the upper
pole of the Batrachian egg) over the yolk (lower pole of the Batrachian
egg), we have an example of the manner in which the primitive involution,
to form the alimentary canal, invariably disappears when the quantity of
yolk in an egg becomes very great.
I believe that in the Dog-fish we have before our eyes one of the steps by
which a direct mode of formation comes to be substituted for an _in_direct
one by involution. We find, in fact, in the Dog-fish, that the cells from
which are derived the mesoblast and hypoblast come to occupy their final
position in the primitive arrangement of the cells during segmentation, and
not by a subsequent and secondary involution.
This change in the mode of formation of the alimentary canal is clearly a
result of change of mechanical conditions from the presence of the large
food-yolk.
Excellent parallels to it will be found amongst the Mollusca. In this class
the presence or absence of food-yolk produces not very dissimilar changes
to those which are produced amongst vertebrates from the same cause.
The continuity of the hypoblast and epiblast at the embryonic rim is a
remnant which, having no meaning or function, except in reference to the
earlier mode of development, is likely to become lost, and in Birds no
trace of it is any longer to be found.
I will not in the present preliminary paper attempt hypothetically to trace
the steps by which the involution gradually disappeared, though I do not
think it would be very difficult to do so. Nor will I attempt to discuss
the question whether the condition with a large amount of food-yolk (as
seems more probable) was twice acquired--once by the Elasmobranchii and
Osseous fishes, and once by Reptiles and Birds--or whether only once, the
Reptiles and Birds being lineal descendants of the Dog-fish.
In reference to the former point, however, I may mention that the
Batrachians and Lampreys are to a certain extent intermediate in condition
between the _Amphioxus_ and the Dog-fishes, since in them the yolk becomes
divided during segmentation into lower layer cells and epiblast, but a
modified involution is still retained, while the Dog-fish may be looked
upon as intermediate between Birds and Batrachians, the continuity at the
hind end between the epiblast and hypoblast being retained by them, though
not the involution.
It may be convenient here to call attention to some of the similarities and
some of the differences which I have not yet spoken of between the
development of Osseous fish and the Dog-fish in the early stages. The
points of similarity are--(1) The swollen edge of the blastoderm. (2) The
embryo-swelling. (3) The embryo-keel. (4) The spreading of the blastoderm
over the yolk-sac from a point corresponding with the position of the
embryo, and not with the centre of the germ. The growth is almost nothing
at that point, and most rapid at the opposite pole of the blastoderm, being
less and less rapid along points of the circumference in proportion to
their proximity to the embryonic swelling. (5) The medullary groove.
In external appearance the early embryos of Dog-fish and Teleostei are very
similar; some of my drawings could almost be substituted for those given by
Oellacher. This similarity is especially marked at the first appearance of
the medullary groove. In the Dog-fish the medullary groove becomes
converted into the medullary canal in the same way as in Birds and all
other vertebrates, except Osseous fishes, where it comes to nothing, and
is, in fact, a rudimentary structure. But in spite of Oellacher's
assertions to the contrary, I am convinced from the similarity of its
position and appearance to the true medullary groove in the Dog-fish, that
the groove which appears in Osseous fishes is the true medullary groove;
although Oellacher and Kuppfer appear to have conclusively proved that it
does not become converted into the medullary canal. The chief difference
between the Dog-fish and Osseous fish, in addition to the point of
difference about the medullary groove, is that the epiblast is in the
Dog-fish a single layer, and not divided into nervous and epidermic layers
as in Osseous fish, and this difference is the more important, since,
throughout the whole period of development till after the commencement of
the formation of the neural canal, the epiblast remains in Dog-fish as a
one-cell-deep layer of cells, and thus the possibility is excluded of any
concealed division into a neural and epidermic layer, as has been supposed
to be the case by Stricker and others in Birds.
_Development of the Embryo._
After the embryo has become definitely established, for some time it grows
rapidly in length, without externally undergoing other important changes,
with the exception of the appearance of two swellings, one on each side of
its tail.
These swellings, which I will call the _Caudal lobes_ (figs. 8 and 9,
_ts_), are also found in Osseous fishes, and have been called by Oellacher
the _Embryonal saum_. They are caused by a thickening of mesoblast on each
side of the hind end of the embryo, at the edge of the embryonic rim, and
form a very conspicuous feature throughout the early stages of the
development of the Dog-fish, and are still more marked in the Torpedo
(Pl. 3, fig. 9). Although from the surface the other changes which are
visible are very insignificant, sections shew that the _notochord_ is
commencing to be formed.
I pointed out that beneath the medullary groove the epiblast and hypoblast
were not separated by any interposed mesoblast. Along the line (where the
mesoblast is deficient) which forms the long axis of the embryo, a rod-like
thickening of the hypoblast appears (Pl. 3, figs. 7_a_ and 7_b_, _ch_ and
_ch´_), first 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, and the separation is then carried backwards. This
thickening of the hypoblast projects up and comes in contact with the
epiblast, and in the later stages with bad (especially chromic-acid)
specimens the line of separation between the epiblast and the thickening
may become a little obscured, and might possibly lead to the supposition
that a structure similar to that which has been called the "_axis cord_"
was present. In all my best (osmic-acid) specimens the line of junction is
quite clear; and any one who is aware how easily two separate masses of
cells may be made indistinguishably to fuse together from simple pressure
will not be surprised to find the occasional obscurity of the line of
junction between the epiblast and hypoblast. In the earlier stage of the
thickening there is never in the osmic-acid preparations any appearance of
fusion except in very badly prepared ones. Its mode of formation will be
quite clear without further description from an inspection of Pl. 3, figs.
7_a_ and 7_b_, _ch_ and _ch´_. Both are taken from one embryo. In
fig. 7_b_, the most anterior of the two, the notochord has become quite
separated from the hypoblast. In fig. 7_a_, _ch_, there is only a very
marked thickening of hypoblast, which reaches up to the epiblast, but the
thickening is still attached to the hypoblast. Had I had space to insert a
drawing of a third section of the same embryo there would only have been a
slight thickening of the hypoblast. In the earlier stage it will be seen,
by referring to figs. 6_a_ and 6_b_, that there is no sign of a thickening
of the hypoblast. My numerous sections (all made from embryos hardened in
osmic acid) shewing these points are so clear that I do not think there can
be any doubt whatever of the notochord being formed as a thickening of the
hypoblast. Two interpretations of this seem possible.
I mentioned that the mesoblast appeared to be primitively formed as two
independent sheets, _split off, so to speak, from the hypoblast_, one on
each side of the middle line of the embryo. If we looked upon the notochord
as a third _median sheet of mesoblast_, split off from the hypoblast
somewhat later than the other two, we should avoid having to admit its
hypoblastic origin.
Professor Huxley, to whom I have shewn my specimens, strongly advocates
this view.
The other possibility is that the notochord is primitively a true
_hypoblastic_ structure which has only by adaptation become an apparently
_mesoblastic_ one in the higher vertebrates. In favour of this view are the
following considerations:
(1) That this is the undoubtedly natural interpretation of the sections.
(2) That the notochord becomes separated from the hypoblast after the
latter has acquired its typical structure, and differs in that respect from
the two lateral sheets of mesoblast, which are formed coincidently with the
hypoblast by a homogeneous mass of cells becoming differentiated into two
distinct layers. (3) That the first mode of looking at the matter really
proves too much, since it is clear that by the same method of reasoning we
could prove the mesoblastic origin of any organ derived from the hypoblast
and budded off into the mesoblast. We would merely have to assert that it
was really a mass of mesoblast budded off from the hypoblast rather later
than the remainder of the mesoblast. Still, it must be admitted that the
first view I have suggested is a possible, not to say a probable one,
though the mode of arguing by which it can be upheld may be rather
dangerous if generally applied. We ought not, however, for that reason
necessarily to reject it in the present case. As Mr Ray Lankester pointed
out to me, if we accept the hypoblastic origin of the notochord, we should
find a partial parallel to it in the endostyle of Tunicates, and it is
perhaps interesting to note in reference to it that the notochord is the
only _unsegmented_ portion of the axial skeleton.
Whether the strong _à priori_ difficulties of the hypoblastic origin of the
notochord are sufficient to counterbalance the natural interpretation of my
sections, cannot, I think, be decided from the single case of the Dog-fish.
It is to be hoped that more complete investigations of the Lamprey, &c.,
may throw further light upon the question.
Whichever view of the primitive origin of the notochord is the true one,
its apparent origin is very instructive as illustrating the possible way in
which an organ might come to change the layer to which it primarily
belonged.
If the notochord is a true mesoblastic structure, it is easy to be seen
how, by becoming separated from the hypoblast a little later than is the
case with the Dog-fish, its mesoblastic origin would become lost; while if,
on the other hand, it is primitively a hypoblastic structure, we see from
higher vertebrates how, by becoming separated from the hypoblast rather
earlier than in the Dog-fish, viz. at the same time as the rest of the
mesoblast, its primitive derivation from the hypoblast has become
concealed.
The view seemingly held by many embryologists of the present day, that an
organ, when it was primitively derived from one layer, can never be
apparently formed in another layer, appears to me both unreasonable on _à
priori_ grounds, and also unsupported by facts.
I see no reason for doubting that the embryo in the earliest periods of
development is as subject to the laws of natural selection as is the animal
at any other period. Indeed, there appear to me grounds for the thinking
that it is more so. The remarkable differences in allied species as to the
amount of food-yolk, which always entail corresponding alterations in the
development--the different modes of segmentation in allied species, such as
are found in the Amphipoda and Isopoda--the suppression of many stages in
freshwater species, which are retained in the allied marine species--are
all instances of modifications due to natural selection affecting the
earliest stages of development. If such points as these can be affected by
natural selection I see no reason why the arrangement of individual cells
(or rather primitive elements) should not also be modified; why, in fact, a
mass of cells which was originally derived from one layer, but in the
course of development became budded off from that layer and entered another
layer, should not by a series of small steps cease ever to be attached to
the original layer, but from the first moment it can be distinguished
should be found as a separate mass in the second layer.
The change of layers will, of course, only take place where some economy is
effected by it. The variations in the mode of development of the nervous
system may probably be explained in this way.
If we admit that organs can undergo changes, as to the primitive layer from
which they are derived, important consequences must follow.
It will, for instance, by no means be sufficient evidence of two organs not
being homologous that they are not developed from the same layer. It
renders the task of tracing out the homologies from development much more
difficult than if the ordinary view of the invariable correspondence of the
three layers throughout the animal kingdom be accepted. Although I do not
believe that this correspondence is invariable or exact, I think that we
both find and should expect to find that it is, roughly speaking, fairly
so.
Thus, the muscles, internal skeleton, and connective tissue are always
placed in the adult between the skin (epidermis) and the epithelium of the
alimentary canal.
We should therefore expect to find them, and, as a matter of fact, we
always do find them, developed from a middle layer when this is present.
The upper layer must always and does always form the epidermis, and
similarly the lower layer or hypoblast must form a part of the epithelium
of the alimentary canal. A full discussion of this question would, however,
lead me too far away from my present subject.
The only other point of interest which I can touch on in this stage is the
commencing closure of the alimentary canal in the region of the head. This
is shewn in Pl. 3, figs. 6_a_, 6_b_, 7_b_, _n.a_. From these figures it
can be seen that the closing does not take place as much by an infolding as
by an ingrowth from the side walls of the alimentary canal towards the
middle line. In this abnormal mode of closing of the alimentary canal we
have again, I believe, an intermediate stage between the mode of formation
of the alimentary canal in the Frog and the typical folding in which occurs
in Birds. There is, however, another point in reference to it which is
still more interesting. The cells to form the ingrowth from the bottom
(ventral) wall of the alimentary canal are derived by a continuous fresh
formation from the yolk, being formed around the nuclei spoken of above
(vide p. 63 et seq.). All my sections shew this with more or less
clearness, especially those a little later than fig. 6_b_, in which the
lower wall of the alimentary canal is nearly completed. This is the more
interesting since, from the mode of formation of the alimentary canal in
the Batrachians, &c., we might expect that the cells from the yolk would
take a share in its formation in the Dog-fish. I have not as yet made out
for certain the share which is taken by these freshly formed cells of the
yolk in the formation of any other organ.
By the completion of its lower wall in the way described, the throat early
becomes a closed tube, its closing taking place before any other important
changes are visible in the embryo from surface views.
A considerable increase in length is attained before other changes than an
increase in depth of the medullary groove and a more complete folding off
of the embryo from the blastoderm take place. The first important change is
the formation of the protovertebræ.
These are formed by the lateral plates of mesoblast, which I said were
equivalent at once to the vertebral and lateral plates in the Bird,
becoming split by transverse divisions into cubical masses.
At the time when this occurs, and, indeed, up till a considerably later
period, the mesoblast is not split into somatopleure and splanchnopleure,
and it is not divided into vertebral and lateral plates. The transverse
lines of division of the protovertebræ do not, however, extend to the outer
edge of the undivided lateral plates.
The differences between this mode of formation of the protovertebræ and
that occurring in Birds are too obvious to require pointing out. I will
speak of them more fully when I have given the whole history of the
protovertebræ of the Dog-fish.
I will only now say that I have had in the early stages to investigate the
formation of the protovertebræ entirely by means of sections, the objects
being too opaque to be otherwise studied.
The next change of any importance is the commencement of the formation of
the head. The region of the head first becomes distinguishable by the
flattening out of the germ at its front end.
The flattened-out portion of the germ grows rapidly, and forms a
spatula-like termination to the embryo (Pl. 3, fig. 8).
In the region of the head the medullary groove is at first totally absent
(vide section, Pl. 3, fig. 8_a_).
Indeed, as can be seen from fig. 8_b_, the laminæ dorsales, so far from
bending up at this stage, actually bend down in the opposite direction.
I am at present quite unable even to form a guess what this peculiar
feature of the brain means. It, no doubt, has some meaning in reference to
the vertebrate ancestry if we could only discover it. The peculiar
spatula-like flattened condition of the head is also (vide _loc. ant.
cit._) apparently found in the Sturgeons; it must therefore almost
undoubtedly be looked upon as not merely an accidental peculiarity.
While these changes have been taking place in the head not less important
changes have occurred in the remainder of the body. In the first place the
two caudal lobes have increased in size, and have become, as it were,
pushed in together, leaving a groove between them (fig. 8, _ts_). They are
very conspicuous objects, and, together with the spatula-like head, give
the whole embryo an almost comical appearance. The medullary canal has by
this time become completely closed in the region of the tail (figs. 8 and
8_b_).
It is still widely open in the region of the back, and, though more nearly
closed again in the neck, is, as I have said, flattened out to nothing in
the head.
The groove[15] between the two caudal lobes must not be confused (as may
easily be done) with the medullary groove, which by the time the former
groove has become conspicuous is a completely closed canal.
Footnote 15: This groove is the only structure which it seems
possible to compare with the so-called "primitive groove" of
Birds. It is, however, doubtful whether they are really
homologous.
The vertebral plates are not divided (vide fig. 7) into a somatopleuric and
splanchnopleuric layer by this stage, except in the region of the head
(vide fig. 8_b_, _pp_), where there is a distinct space between the two
layers, which is undoubtedly homologous with the pleuro-peritoneal cavity
of the hinder portion of the body.
It is probably the same cavity which Oellacher (_loc. cit._) calls in
Osseous fishes the pericardial cavity. In the Dog-fish, at least, it has no
connection with the pericardium. Of its subsequent history I shall say a
few words when I come to speak of the later stages.
The embryo does not take more than twenty-four hours in passing from this
stage, when the head is a flat plate, to the stage when the whole neural
canal (including the region of the head) is closed in. The other changes,
in addition to the closing in of the neural canal, are therefore somewhat
insignificant. The folding off of the embryo from the germ has, however,
progressed considerably, and a portion of the hind gut is closed in below.
This is accomplished, not by a tail-fold, as in Birds, but by two lateral
folds, which cause the sides of the body to meet and coalesce below. At the
extreme hind end, where the epiblast is continuous with the hypoblast, the
lateral folds turn round, so to speak, and become continuous with the
medullary folds, so that when the various folds meet
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