The Science and Philosophy of the Organism
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Author: Hans Driesch
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Transcriber’s notes:
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text by =equal signs=. Subscripts are indicated by _underscores_ (e.g.
*a*_1 and *b*_1 in Fig.5 caption) and superscripts by ^ (e.g. *a*^1. in
Fig. 9 caption).
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silently (e.g. invividual-->individual, hyberbola-->hyperbola) but
inconsistent use of the ligature æ/ae (e.g. palæontology/palaeontology),
inconsistent use of alternative spellings (e.g. learned/learnt), and
occasional inconsistencies of hyphenation have been left as in the
original. Minor punctuation typos have been corrected silently (e.g.
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heading Pt.I-B-3 which contains an extraneous α.
In Figure 12 caption, multiple ditto marks have been replaced by the
relevant text for greater clarity.
THE SCIENCE AND PHILOSOPHY
OF THE ORGANISM
AGENTS
America The Macmillan Company
64 & 66 Fifth Avenue, New York
Australasia The Oxford University Press, Melbourne
Canada The Macmillan Company of Canada, Ltd.
27 Richmond Street West, Toronto
India Macmillan & Company, Ltd.
Macmillan Building, Bombay
309 Bow Bazaar Street, Calcutta
THE
SCIENCE AND PHILOSOPHY
OF THE ORGANISM
THE GIFFORD LECTURES DELIVERED BEFORE
THE UNIVERSITY OF ABERDEEN
IN THE YEAR 1907
BY
HANS DRIESCH, Ph.D.
HEIDELBERG
LONDON
ADAM AND CHARLES BLACK
1908
*All rights reserved*
PREFACE
This work is not a text-book of theoretical biology; it is a systematic
presentment of those biological topics which bear upon the true
philosophy of nature. The book is written in a decidedly subjective
manner, and it seems to me that this is just what “Gifford Lectures”
ought to be. They ought never to lose, or even try to lose, their
decidedly personal character.
My appointment as Gifford Lecturer, the news of which reached me in
February 1906, came just at the right moment in the progress of my
theoretical studies. I had always tried to improve my previous books by
adding notes or altering the arrangement; I also had left a good deal of
things unpublished, and thus I often hoped that I might have occasion
to arrange for a new, improved, and enlarged edition of those books.
This work then is the realisation of my hopes; it is, in its way, a
definitive statement of all that I have to say about the Organic.
The first volume of this work, containing the lectures for 1907--though
the division into “lectures” has not been preserved--consists of Parts
I. and II. of Section A, “The Chief Results of Analytical Biology.”
It gives in Part I. a shortened, revised, and, as I hope, improved
account of what was published in my *Analytische Theorie der organischen
Entwickelung* (1894), *Die Localisation morphogenetischer Vorgänge;
ein Beweis Vitalistischen Geschehens* (1899), and *Die organischen
Regulationen* (1901), though for the professed biologist the two
last-named books are by no means superseded by the new work. Part II.
has never been published in any systematic form before, though there are
many remarks on Systematics, Darwinism, etc., in my previous papers.
The second volume--to be published in the autumn, after the delivery of
the 1908 lectures--will begin with the third and concluding part of the
scientific section, which is a very carefully revised and rearranged
second edition of my book, *Die “Seele” als elementarer Naturfactor*
(1903). The greater part of this volume, however, will be devoted to the
“Philosophy of the Organism,” *i.e.* Section B, which, in my opinion,
includes the most important parts of the work.
Some apology is needed for my presuming to write in English. I was
led to do so by the conviction, mistaken perhaps, that the process of
translation would rob the lectures of that individual and personal
character which, as I said before, seems to me so much to be desired. I
wished nothing to come between me and my audience. I accordingly wrote
my manuscript in English, and then submitted it to linguistic revision
by such skilled aid as I was able to procure at Heidelberg. My reviser
tells me that if the result of his labours leaves much to be desired,
it is not to be wondered at, but that, being neither a biologist nor a
philosopher, he has done his best to make me presentable to the English
reader. If he has failed in his troublesome task, I know that it is not
for want of care and attention, and I desire here to record my sense of
indebtedness to him. He wishes to remain anonymous, but I am permitted
to say that, though resident in a foreign university, he is of Scottish
name and English birth.
My gratitude to my friends at Aberdeen, in particular to Professor and
Mrs. J. A. Thomson, for their hospitality and great kindness towards me
cannot be expressed here; they all know that they succeeded in making me
feel quite at home with them.
I am very much obliged to my publishers, Messrs. A. and C. Black, for
their readiness to fulfil all my wishes with respect to publication.
The lectures contained in this book were written in English by a
German and delivered at a Scottish university. Almost all of the ideas
discussed in it were first conceived during the author’s long residence
in Southern Italy. Thus this book may be witness to the truth which,
I hope, will be universally recognised in the near future--that all
culture, moral and intellectual and aesthetic, is not limited by the
bounds of nationality.
HANS DRIESCH.
Heidelberg, *2nd January 1908*.
CONTENTS OF THE FIRST VOLUME
THE PROGRAMME
PAGE
On Lord Gifford’s Conception of “Science” 1
Natural Sciences and “Natural Theology” 3
Our Philosophical Basis 5
On Certain Characteristics of Biology as a Science 9
The Three Different Types of Knowledge about Nature 13
General Plan of these Lectures 15
General Character of the Organic Form 19
SECTION A.--THE CHIEF RESULTS OF ANALYTICAL BIOLOGY
PART I.--THE INDIVIDUAL ORGANISM WITH REGARD TO FORM AND METABOLISM
*A.* ELEMENTARY MORPHOGENESIS--
Evolutio and Epigenesis in the old Sense 25
The Cell 27
The Egg: its Maturation and Fertilisation 31
The First Developmental Processes of Echinus 33
Comparative Embryology 44
The First Steps of Analytical Morphogenesis 45
The Limits of Pure Description in Science 50
*B.* EXPERIMENTAL AND THEORETICAL MORPHOGENESIS--
1. THE FOUNDATIONS OF THE PHYSIOLOGY OF DEVELOPMENT.
“EVOLUTIO” AND “EPIGENESIS” 52
The Theory of Weismann 52
Experimental Morphology 56
The Work of Wilhelm Roux 58
The Experiments on the Egg of the Sea-urchin 59
On the Intimate Structure of the Protoplasm of the Germ 65
On some Specificities of Organisation in Certain Germs 70
General Results of the First Period of “Entwickelungsmechanik” 71
Some New Results concerning Restitutions 74
2. ANALYTICAL THEORY OF MORPHOGENESIS 76
α. THE DISTRIBUTION OF MORPHOGENIC POTENCIES 76
Prospective Value and Prospective Potency 76
The Potencies of the Blastomeres 79
The Potencies of Elementary Organs in General 80
Explicit and Implicit Potencies: Primary and Secondary
Potencies 83
The Morphogenetic Function of Maturation in the Light of
Recent Discoveries 85
The Intimate Structure of Protoplasm: Further Remarks 88
The Neutrality of the Concept of “Potency” 89
β. THE “MEANS” OF MORPHOGENESIS 89
β′. The Internal Elementary Means of Morphogenesis 90
Some Remarks on the Importance of Surface Tension in
Morphogenesis 91
On Growth 93
On Cell-division 94
β″. The External Means of Morphogenesis 95
The Discoveries of Herbst 96
γ. THE FORMATIVE CAUSES OR STIMULI 99
The Definition of Cause 99
Some Instances of Formative and Directive Stimuli 102
δ. THE MORPHOGENETIC HARMONIES 107
ε. ON RESTITUTIONS 110
A few Remarks on Secondary Potencies and on Secondary
Morphogenetic Regulations in General 110
The Stimuli of Restitutions 113
3. THE PROBLEM OF MORPHOGENETIC LOCALISATION: THE
THEORY OF THE HARMONIOUS-EQUIPOTENTIAL SYSTEM--FIRST
PROOF OF THE AUTONOMY OF LIFE 118
The General Problem 118
The Morphogenetic “System” 119
The “Harmonious-equipotential System” 122
Instances of “Harmonious-equipotential Systems” 126
The Problem of the Factor *E* 132
No Explanation offered by “Means” or “Formative Stimuli” 132
No Explanation offered by a Chemical Theory of Morphogenesis 134
No Machine Possible Inside the Harmonious Systems 138
The Autonomy of Morphogenesis proved 142
“Entelechy” 143
Some General Remarks on Vitalism 145
The Logic of our First Proof of Vitalism 146
4. ON CERTAIN OTHER FEATURES OF MORPHOGENESIS ADVOCATING
ITS AUTONOMY 150
Harmonious-equipotential Systems formed by Wandering Cells 151
On Certain Combined Types of Morphogenetic Systems 153
The “Morphaesthesia” of Noll 157
Restitutions of the Second Order 158
On the “Equifinality” of Restitutions 159
Remarks on “Retro-Differentiation” 163
*C.* ADAPTATION--
INTRODUCTORY REMARKS ON REGULATIONS IN GENERAL 165
1. MORPHOLOGICAL ADAPTATION 168
The Limits of the Concept of Adaptation 168
Adaptations to Functional Changes from Without 172
True Functional Adaptation 176
Theoretical Conclusions 179
2. PHYSIOLOGICAL ADAPTATION 184
Specific Adaptedness *not* “Adaptation” 186
Primary and Secondary Adaptations in Physiology 188
On Certain Pre-requisites of Adaptations in General 189
On Certain Groups of Primary Physiological Adaptations 190
General Remarks on Irritability 190
The Regulation of Heat Production 193
Primary Regulations in the Transport of Materials and
Certain Phenomena of Osmotic Pressure 194
Chromatic Regulations in Algae 197
Metabolic Regulations 198
Immunity the only Type of a Secondary Physiological
Adaptation 204
No General Positive Result from this Chapter 209
A few Remarks on the Limits of Regulability 212
*D.* INHERITANCE. SECOND PROOF OF THE AUTONOMY OF LIFE--
The Material Continuity in Inheritance 214
On Certain Theories which Seek to Compare Inheritance to Memory 216
The Complex-Equipotential System and its Rôle in Inheritance 219
The Second Proof of Life-Autonomy. Entelechy at the Bottom
of Inheritance 224
The Significance of the Material Continuity in Inheritance 227
The Experimental Facts about Inheritance 228
The Rôle of the Nucleus in Inheritance 233
Variation and Mutation 237
*CONCLUSIONS FROM THE FIRST MAIN PART OF THESE LECTURES* 240
PART II.--SYSTEMATICS AND HISTORY
*A.* THE PRINCIPLES OF SYSTEMATICS--
Rational Systematics 243
Biological Systematics 246
*B.* THE THEORY OF DESCENT--
1. GENERALITIES 250
The Covert Presumption of all Theories of Descent 253
The Small Value of Pure Phylogeny 255
History and Systematics 257
2. THE PRINCIPLES OF DARWINISM 260
Natural Selection 261
Fluctuating Variation the Alleged Cause of Organic Diversity 264
Darwinism Fails all along the Line 269
3. THE PRINCIPLES OF LAMARCKISM 271
Adaptation as the Starting-Point 272
The Active Storing of Contingent Variations as a
Hypothetic Principle 273
Criticism of the “Inheritance of Acquired Characters”
assumed by Lamarckism 275
Other Principles Wanted 281
Criticism of the Hypothesis of Storing and Handing Down
Contingent Variations 282
4. THE REAL RESULTS AND THE UNSOLVED PROBLEMS OF TRANSFORMISM 290
5. THE LOGICAL VALUE OF THE ORGANIC FORM ACCORDING TO THE
DIFFERENT TRANSFORMISTIC THEORIES 293
The Organic Form and Entelechy 294
*C.* THE LOGIC OF HISTORY 297
1. THE POSSIBLE ASPECTS OF HISTORY 299
2. PHYLOGENETIC POSSIBILITIES 304
3. THE HISTORY OF MANKIND 306
Cumulations in Human History 308
Human History not an “Evolution” 311
The Problem of the “Single” as such 315
*CONCLUSIONS ABOUT SYSTEMATICS AND HISTORY IN GENERAL* 322
THE PROGRAMME
ON LORD GIFFORD’S CONCEPTION OF “SCIENCE”
This is the first time that a biologist has occupied this place; the
first time that a biologist is to try to carry out the intentions of the
noble and high-minded man to whom this lectureship owes its foundation.
On such an occasion it seems to be not undesirable to inquire what Lord
Gifford’s own opinions about natural science may have been, what place
in the whole scheme of human knowledge he may have attributed to those
branches of it which have become almost the centre of men’s intellectual
interest.
And, indeed, on studying Lord Gifford’s bequest with the object of
finding in it some reference to the natural sciences, one easily notes
that he has assigned to them a very high place compared with the other
sciences, at least in one respect: with regard to their methods.
There is a highly interesting passage in his will which leaves no doubt
about our question. After having formally declared the foundation of
this lectureship “for Promoting, Advancing, Teaching and Diffusing
the study of Natural Theology in the widest sense of that term,” and
after having arranged about the special features of the lectures, he
continues: “I wish the lecturers to treat their subject as a strictly
natural science, the greatest of all possible sciences, indeed, in one
sense, the only science, that of Infinite Being.... I wish it considered
just as astronomy or chemistry is.”
Of course, it is not possible to understand these words of Lord
Gifford’s will in a quite literal sense. If, provisionally, we call
“natural theology” the ultimate conclusions which may be drawn from a
study of nature in connection with all other results of human sciences,
there cannot be any doubt that these conclusions will be of a rather
different character from the results obtained in, say, the special field
of scientific chemistry. But, nevertheless, there are, I think, two
points of contact between the wider and the narrower field of knowledge,
and both of them relate to method. Lord Gifford’s own phrase, “Infinite
Being,” shows us one of these meeting-points. In opposition to history
of any form, natural sciences aim at discovering such truths as are
independent of special time and of special space, such truths as are
“ideas” in the sense of Plato; and such eternal results, indeed, always
stand in close relation to the ultimate results of human knowledge
in general. But besides that there is still another feature which
may be common both to “natural theology” and to the special natural
sciences, and which is most fully developed in the latter: freedom from
prepossessions. This, at least, is an ideal of all natural sciences;
I may say it is *the* ideal of them. That it was this feature which
Lord Gifford had in view in his comparison becomes clear when we read
in his will that the lectures on natural theology are to be delivered
“without reference to or reliance upon any supposed special exceptional
or so-called miraculous revelation.”
So we might say that both in their logical and their moral methods,
natural sciences are to be the prototype of “Natural Theology” in Lord
Gifford’s sense.
NATURAL SCIENCES AND “NATURAL THEOLOGY”
But now let us study in a more systematic manner the possible relations
of the natural sciences to natural theology as a science.
How is it possible for a natural scientist to contribute to the science
of the highest and ultimate subject of human knowledge?
Almost all natural sciences have a sort of naïveté in their own spheres;
they all stand on the ground of what has been called a naïve realism,
as long as they are, so to say, at home. That in no way prejudices
their own progress, but it seems to stand in the way of establishing
contact with any higher form of human knowledge than themselves.
One may be a first-rate organic chemist even when looking upon the
atoms as small billiard balls, and one may make brilliant discoveries
about the behaviour of animals even when regarding them in the most
anthropomorphic manner--granted that one is a good observer; but it
can hardly be admitted that our chemist would do much to advance the
theory of matter, or our biologist to solve the problem of the relations
between body and mind.
It is only by the aid of philosophy, or I would rather say by keeping
in constant touch with it, that natural sciences are able to acquire
any significance for what might be called *the* science of nature in the
most simple form. Unhappily the term “natural philosophy” is restricted
in English to theoretical physics. This is not without a high degree of
justification, for theoretical physics has indeed lost its naïveté and
become a philosophy of nature; but it nevertheless is very unfortunate
that this use of the term “natural philosophy” is established in
this country, as we now have no proper general term descriptive of a
natural science that is in permanent relation to philosophy, a natural
science which does not use a single concept without justifying it
epistemologically, *i.e.* what in German, for instance, would simply be
called “Naturphilosophie.”
Let us call it philosophy of nature; then we may say that only by
becoming a true philosophy of nature are natural sciences of all sorts
able to contribute to the highest questions which man’s spirit of
inquiry can suggest.
These highest questions themselves are the outcome of the combination
of the highest results of all branches of philosophy, just as our
philosophy of nature originated in the discussion of the results of
all the separate natural sciences. Are those highest questions not
only to be asked, are they to be also solved? To be solved in a way
which does not exceed the limits of philosophy as the domain of actual
understanding?
The beginning of a long series of studies is not the right place to
decide this important question; and so, for the present certainly,
“natural theology” must remain a problem. In other words: it must
remain an open question at the beginning of our studies, whether after
all there can be any final general answer, free from contradictions,
applicable to the totality of questions asked by all the branches of
philosophy.
But let us not be disturbed by this problematic entrance to our studies.
Let us follow biology on its own path; let us study its transition from
a “naïve” science to a real branch of the philosophy of nature. In this
way we perhaps shall be able to understand what its part may be in
solving what can be solved.
That is to be our subject.
OUR PHILOSOPHICAL BASIS
We call *nature* what is given to us in space.
Of course we are not obliged in these lectures to discuss the
psychological and epistemological problems of space with its three
dimensions, nor are we obliged to develop a general theory of reality
and its different aspects. A few epistemological points will be
considered later at proper times, and always in connection with results
of theoretical biology.
At present it must suffice to say that our general philosophical point
of view will be idealistic, in the critical meaning of the word. The
universe, and within the universe nature, in the sense just defined,
is my phenomenon. That is what I know. I know nothing more, either
positively or negatively; that is to say, I do not know that the world
is *only my* phenomenon, but, on the other hand, I know nothing about
its “absolute reality.” And more, I am not even able to describe in
intelligible words what “absolute reality” might mean. I am fully
entitled to state: the universe *is* as truly as I am--though in
a somewhat different sense of “being”--and I *am* as truly as the
universe is; but I am not entitled to state anything beyond these
two corresponding phrases. You know that, in the history of European
philosophy at least, Bishop Berkeley was the first clearly to outline
the field of idealism.
But my phenomenon--the world, especially nature--consists of elements
of two different kinds: some of them are merely passive, some of them
contain a peculiar sort of activity in themselves. The first are
generally called sensations, but perhaps would be better called elements
or presentations; the others are forms of construction, and, indeed,
there is an active element embraced in them in this sense, that they
allow, by their free combination, the discovery of principles which
are not to be denied, which must be affirmed, whenever their meaning
is understood. You know that I am speaking here of what are generally
called categories and synthetic judgments *a priori*, and that it was
Kant who, on the foundations laid by Locke, Hume, and Leibnitz, first
gave the outlines of what may be called the real system of critical
philosophy. Indeed, our method will be to a great extent Kantian, though
with certain exceptions; it is to be strictly idealistic, and will not
in the Kantian way operate with things in themselves; and it regards
the so-called “synthetic judgment *a priori*” and the problem of the
relation between categorical principles and experience in a somewhat
different manner. We think it best to define the much disputed concept
“*a priori*” as “independent of the *amount* of experience”; that is
to say, all categories and categorical principles are brought to my
consciousness by that fundamental event which is called experience, and
therefore are not independent of it, but they are not inferences from
experience, as are so-called empirical laws. We almost might say that we
only have to be reminded of those principles by experience, and, indeed,
we should not, I think, go very far wrong in saying that the Socratic
doctrine, that all knowledge is recollection, holds good as far as
categories and categorical principles are in question.
But enough at present about our general philosophy.
As to the philosophy of nature, there can be no doubt that, on the basis
of principles like those we have shortly sketched, its ultimate aim must
be to co-ordinate everything in nature with terms and principles of the
categorical style. The philosophy of nature thus becomes a system; a
system of which the general type is afforded by the innate constructive
power of the Ego. In this sense the Kantian dictum remains true, that
the Ego prescribes its own laws to nature, though, of course, “nature,”
that is, what is given in space, must be such as to permit that sort of
“prescription.”
One often hears that all sciences, including the science of sciences,
philosophy, have to find out what is true. What, then, may be called
“true” by an idealistic philosopher, for whom the old realistic formula
of the conformity between knowledge and the object cannot have any
meaning? Besides its ordinary application to simple facts or to simple
judgments, where the word truth only means absence of illusion or no
false statement, truth can be claimed for a philosophical doctrine or
for a system of such doctrines only in the sense that there are no
contradictions amongst the parts of the doctrine or of the system
themselves, and that there are no features in them which impel our
categorical Ego to further analysis.
Those of you who attended Professor Ward’s lectures on “Naturalism and
Agnosticism,” or who have read his excellent book on that subject, will
know what the aims of a theory of matter are. You will also be aware
that, at present, there does not exist any theory of matter which can
claim to be “true”; there are contradictions in every theory of matter,
and, moreover, there are always some points where we are obliged to ask
for further information and receive no answer. Experience here has not
yet aroused all the categorical functions which are needed in order to
form one unity out of what seem to be incompatibilities at the present
day. Why is that? Maybe because experience is not yet complete in this
field, but maybe also because the whole subject is so complicated
that it takes much time to attach categorical functions to what is
experienced.
But it is not our object here to deal either with epistemology proper
or with ontology: a full analysis of biological facts is our problem.
Why, then, all these introductions? why all these philosophical sketches
in fields of knowledge which have quite another relation to philosophy
than biology has? Biology, I hear some one say, is simply and solely
an empirical science; in some sense it is nothing but applied physics
and chemistry, perhaps applied mechanics. There are no fundamental
principles in biology which could bring it in any close contact with
philosophy. Even the one and only principle which might seem to be
an innate principle of our experience about life, the principle of
evolution, is only a combination of more simple factors of the physical
and chemical type.
It will be my essential endeavour to convince you, in the course of
these lectures, that such an aspect of the science of biology is wrong;
that biology is an elemental natural science in the true sense of the
word.
But if biology is an elemental science, then, and only then, it stands
in close relations to epistemology and ontology--in the same relations
to them, indeed, as every natural science does which deals with true
elements of nature, and which is willing to abandon naïve realism and
contribute its share to the whole of human knowledge.
And, therefore, a philosophical sketch is not out of place at the
beginning of lectures on the Philosophy of the Organism. We may be
forced, we, indeed, shall be forced, to remain for some time on the
ground of realistic empiricism, for biology has to deal with very
complicated experiences; but there will be a moment in our progress when
we shall enter the realm of the elemental ontological concepts, and
in that very moment our study of life will have become a part of real
philosophy. It was not without good reasons, therefore, that I shortly
sketched, as a sort of introduction to my lectures, the general point of
view which we shall take with regard to philosophical questions, and to
questions of the philosophy of nature in particular.
ON CERTAIN CHARACTERISTICS OF BIOLOGY AS A SCIENCE
Biology is the science of life. Practically, all of you know what a
living being is, and therefore it is not necessary to formulate a
definition of life, which, at the beginning of our studies, would be
either provisional and incomplete, or else dogmatic. In some respects,
indeed, a definition should rather be the end of a science than its
opening.
We shall study the phenomena of living organisms analytically, by the
aid of experiment; our principal object will be to find out laws in
these phenomena; such laws will then be further analysed, and precisely
at that point we shall leave the realm of natural science proper.
Our science is the highest of all natural sciences, for it embraces as
its final object the actions of man, at least in so far as actions also
are phenomena observable on living bodies.
But biology is also the most difficult of all natural sciences, not
only from the complexity of the phenomena, which it studies, but in
particular for another reason which is seldom properly emphasised, and
therefore will well repay us for a few words devoted to it.
Except so far as the “elements” of chemistry come into account, the
experimenter in the inorganic fields of nature is not hampered by the
specificity of composite objects: he makes all the combinations he
wants. He is always able to have at his disposal red rays of a desired
wave length when and where he wants, or to have, at a given time and
place, the precise amount of any organic compound which he wishes to
examine. And he forces electricity and electromagnetism to obey his
will, at least with regard to space, time, and intensity of their
appearance.
The biologist is not able to “make” life, as the physicist has made red
rays or electromagnetism, or as the chemist has made a certain compound
of carbon. The biologist is almost always in that strange plight in
which the physicist would be if he always had to go to volcanoes in
order to study the conductivity of heat, or if he had to wait for
thunderstorms in order to study electricity. The biologist is dependent
on the specificity of living objects as they occur in nature.
A few instances may show you what great inconveniences may hence arise
to impede practical biological research. We later on shall have to deal
with experiments on very young embryos: parts of the germ will have
to be destroyed in order to study what will happen with the rest. Now
almost all germs are surrounded by a membrane; this membrane has to be
detached before any operation is possible. But what are we to do if it
is not possible to remove the membrane without killing the embryo? Or
what if, as for instance in many marine animals, the membrane may be
removed but the germs are killed by contact with sea-water? In both
cases no experiments at all will be possible on a sort of germ which
otherwise, for some special circumstances of its organisation, might
have given results of importance. These results become impossible for
only a practical, for a very secondary reason; but enough: they are
impossible, and they might have thrown light on problems which now
must remain problems. Quite the same thing may occur in experiments
on physiology proper or functional physiology: one kind of animals
survives the operation, the other kind does not, and therefore, for
merely extrinsic reasons, the investigations have to be restricted to
the first, though the second might have given more important results.
And thus the biological experimenter always finds himself in a sort of
dependence on his subjects, which can hardly be called pleasant. To a
great extent the comparatively slow advance of biological sciences is
due to this very fact: the unalterable specific nature of biological
material.
But there is still another feature of biology dependent on the same
fact. If a science is tied down to specific objects in every path it
takes, it first, of course, has to know all about those objects, and
that requires nothing else but plain description. We now understand why
pure description, in the most simple sense of the word, takes up such an
enormous part of every text-book of biological science. It is not only
morphology, the science of form, that is most actively concerned with
description; physiology also, in its present state, is pure description
of what the functions of the different parts of the body of animals and
plants actually *are*, at least for about nine-tenths of its range. It
seems to me important to press this point very emphatically, since we
often hear that physiology is from the very beginning a much higher sort
of knowledge than morphology, inasmuch as it is rational. That is not at
all true of the beginning of physiology: what the functions of the liver
or of the root are has simply to be described just as the organisation
of the brain or of the leaf, and it makes no difference logically that
one species of description has to use the experimental method, while the
other has not. The experiment which only discovers what happens here or
what happens there, possesses no kind of logical superiority over pure
description at all.
But there will be another occasion in our lectures to deal more fully
with the logic of experiment and with the differences of descriptive
knowledge and real rational science.
THE THREE DIFFERENT TYPES OF KNOWLEDGE ABOUT NATURE
Natural sciences cannot originate before the given phenomena of nature
have been investigated in at least a superficial and provisional manner,
by and for the practical needs of man. But as soon as true science
begins in any limited field, dealing, let us say, with animals or with
minerals, or with the properties of bodies, it at once finds itself
confronted by two very different kinds of problems, both of them--like
all “problems”--created in the last resort by the logical organisation
of the human mind, or, to speak still more correctly, of the Ego.
In any branch of knowledge which practical necessities have separated
from others, and which science now tries to study methodically, there
occur general sequences in phenomena, general orders of events. This
uniformity is revealed only gradually, but as soon as it has shown
itself, even in the least degree, the investigator seizes upon it. He
now devotes himself chiefly, or even exclusively, to the generalities in
the sequences of all changes. He is convinced that there must be a sort
of most general and at the same time of most universal connection about
all occurrences. This most universal connection has to be found out; at
least it will be the ideal that always will accompany the inquiring mind
during its researches. The “law of nature” is the ideal I am speaking
about, an ideal which is nothing less than one of the postulates of the
possibility of science at all.
Using for our purposes a word which has been already introduced into
terminology by the philosopher Windelband, though in a somewhat
different sense, we shall call that part of every branch of natural
sciences which regards the establishment of a law of nature as its
ideal, “nomothetic,” *i.e.* “law-giving.”
But while every natural science has its nomothetic side, it also has
another half of a very different kind. This second half of every natural
science does not care for the same general, the same universal, which
is shown to us in every event in a different and specified kind: it
is diversity, it is specification, that constitutes the subject of
its interest. Its aim is to find a sufficient reason for the types of
diversities, for the types of specifications. So in chemistry there
has been found a systematic order in the long series of the compounds
and of the elements; crystallography also has its different systems of
crystals, and so on.
We have already employed the word by which we shall designate this
second half of every natural science: it is the “systematic” side of
science.
Nomothetic work on the one side and systematics on the other do, in
fact, appear in every natural science, and besides them there are no
other main parts. But “science” as a whole stands apart from another
aspect of reality which is called “history.” History deals with
particulars, with particular events at such and such a place, whilst
science always abstracts from the particular, even in its systematic
half.[1]
[1] Windelband (*Geschichte und Naturwissenschaft*, 3 Auflage, 1904)
gives the name “nomothetic” to the whole of our “science” and calls the
method of history “idiographic.” We thought it better to establish three
fundamental types of all possible branches of knowledge.
GENERAL PLAN OF THESE LECTURES
Turning now to a sort of short outline of what is to be discussed
in the whole of our future lectures, this summer and next, it seems
clear, without further analysis, that biology as a science has its
nomothetic and its systematic part also; respiration and assimilation,
for instance, have proved to be types of natural laws among living
phenomena, and that there is a “system” of animals and plants is too
commonly known to require further explanation here. Therefore we might
study first biological laws, and after that biological systematics, and
in the third place perhaps biological history. But that would hardly
correspond to the philosophical aims of our lectures: our chief object
is not biology as a regular science, as treated in text-books and in
ordinary university lectures; our chief object is the Philosophy of the
Organism, as aided and supported by scientific biology. Therefore a
general acquaintance with biology must be assumed in these lectures, and
the biological materials must be arranged according to their bearing on
further, that is on philosophical, analysis.
That will be done, not, of course, to the extent of my regarding every
one of my audience as a competent biologist; on the contrary, I shall
explain most fully all points of biology proper, and even of the most
simple and descriptive kind of biology, which serve as bases for
philosophical analysis. But I shall do so only if they indeed do serve
as such bases. All our biology will be not for its own sake, but for the
sake of philosophy.
Whilst regarding the whole of the biological material with such aims,
it seems to me best to arrange the properly scientific material which
is to be the basis of my discussions, not along the lines which biology
as an independent science would select,[2] but to start from the three
different kinds of fundamental phenomena which living bodies offer to
investigation, and to attach all systematics exclusively to one of them.
For there will not be very much for philosophy to learn from biological
systematics at present.
[2] See J. Arth. Thomson, *The Science of Life*, London, 1899.
Life is unknown to us except in association with bodies: we only know
living bodies and call them organisms. It is the final object of all
biology to tell us what it ultimately means to say that a body is
“living,” and in what sorts of relation body and life stand one to the
other.
But at present it is enough to understand the terms “body” and “living”
in the ordinary and popular sense.
Regarding living bodies in this unpretentious manner, and recollecting
what the principal characters are of all bodies we know as living ones,
we easily find that there are three features which are never wanting
wherever life in bodies occurs. All living bodies are specific as to
form--they “have” a specific form, as we are accustomed to say. All
living bodies also exhibit metabolism; that is to say, they stand in a
relation of interchange of materials with the surrounding medium, they
take in and give out materials, but their form can remain unchanged
during these processes. And, in the last place, we can say that all
living bodies move; though this faculty is more commonly known among
animals only, even elementary science teaches the student that it also
belongs to plants.
Therefore we may ask for “laws of nature” in biology about form, about
metabolism, and about movements. In fact, it is according to this scheme
that we shall arrange the materials of the biological part of our
lectures, though, as we cannot regard the three divisions as equally
important in their bearing on our ultimate purposes, we shall not treat
them quite on equal terms. It will appear that, at least in the present
state of science, the problems of organic form and of organic movement
have come into much closer relation to philosophical analysis than have
most of the empirical data on metabolism.
It is *form* particularly which can be said to occupy the very centre
of biological interest; at least it furnishes the foundation of all
biology. Therefore we shall begin our scientific studies with a full and
thorough analysis of form. The science of living forms, later on, will
afford us a key to study metabolism proper with the greatest advantage
for our philosophical aims, and therefore the physiology of what is
usually called the vegetative functions will be to us a sort of appendix
to our chapters on form; only the theory of a problematic “living
substance” and of assimilation in the most general meaning of the word
will be reserved for the philosophical part; for very good reasons,
as I hope to show. But our chapters on the living forms will have yet
another appendix besides the survey of the physiology of metabolism.
Biological systematics almost wholly rests on form, on “morphology”; and
what hitherto has been done on the metabolical side of their problems,
consists of a few fragments, which are far from being an equivalent to
the morphological system; though, of course it must be granted that,
logically, systematics, in our general meaning of the word, as the sum
of problems about the typically different and the specific, may be
studied on the basis of each one of the principal characteristics of
living bodies, not only on that of their forms. Therefore, systematics
is to be the second appendix to the chief part of our studies in
morphology, and systematics, in its turn, will later on lead us to
a short sketch of the historical side of biology, to the theory of
evolution in its different forms, and to the logic of history in general.
So far will our programme be carried out during this summer. Next year
the theory of movements will conclude our merely scientific analysis,
and the remaining part of the course next summer will be devoted to the
philosophy of living nature. I hope that nobody will be able to accuse
our philosophy of resting on unsound foundations. But those of you, on
the other hand, who would be apt to regard our scientific chapters as a
little too long compared with their philosophical results, may be asked
to consider that a small clock-tower of a village church is generally
less pretentious but more durable than the campanile of San Marco has
been.
Indeed, these lectures will afford more “facts” to my hearers, than
Gifford Lectures probably have done, as a rule. But how could that
be otherwise on the part of a naturalist? Scientific facts are the
material that the philosophy of nature has to work with, but these
facts, unfortunately, are not as commonly known as historical facts,
for instance, generally are; and they must be known, in order that a
philosophy of the organism may be of any value at all, that it may be
more than a mere entertainment.
Goethe once said, that even in so-called facts there is more “theory”
than is usually granted; he apparently was thinking of what might be
called the ultimate or the typical facts in science. It is with such
typical or ultimate facts, of course, that we must become acquainted if
our future philosophy is to be of profit to us.
Certainly, there would be nothing to prevent us from arranging our
materials in a manner exactly the reverse of that which we shall adopt;
we could begin with a general principle about the organic, and could
try to deduce all its special features from that principle, and such a
way perhaps would seem to be the more fascinating method of argument.
But though logical it would not be psychological, and therefore would
be rather unnatural. And thus our *most* general principle about the
organic will not come on the scene before the eighteenth of these twenty
lectures, although it is not a mere inference or deduction from the
former lectures: it will be a culmination of the whole, and we shall
appreciate its value the better the more we know what that whole really
is.
GENERAL CHARACTER OF THE ORGANIC FORM
Our programme of this year, it was said, is to be devoted wholly
to organic forms, though one of its appendixes, dealing with some
characteristics of the physiology of metabolism, will lead us on to a
few other phenomena. What then are the essentials of a living form, as
commonly understood even without a special study of biology?
Living bodies are not simple geometrical forms, not, like crystals,
merely a typical arrangement of surfaces in space, to be reduced
theoretically, perhaps, to an arrangement of molecules. Living bodies
are typically combined forms; that is to say, they consist of simpler
parts of different characters, which have a special arrangement with
regard to one another; these parts have a typical form of their own and
may again be combinations of more simple different parts. But besides
that, living bodies have not always the same typically combined form
during the whole of their life: they become more complicated as they
grow older; they all begin from one starting point, which has little
form at all, viz., the egg. So the living form may be called a “genetic
form,” or a form considered as a process, and therefore *morphogenesis*
is the proper and adequate term for the science which deals with the
laws of organic forms in general; or, if you prefer not to use the
same word both for a science and for the subjects of that science, the
*physiology of morphogenesis*.
Now there are different branches of the physiology of morphogenesis or
physiology of form. We may study, and indeed we at first shall study,
what are the laws of the morphogenetic processes leading from the egg
to the adult: that may be properly called physiology of development.
But living forms are not only able to originate in one unchangeable
way: they may restore themselves, if disturbed, and thus we get the
physiology of restoration or restitution as a second branch of the
science of morphogenesis. We shall draw very important data, some of the
foundations indeed of our philosophical discussions, from the study of
such restitutions. Besides that, it is to them that our survey of the
problems of the physiology of metabolism is to be appended.
Living forms not only originate from the egg and are able to restore
themselves, they also may give origin to other forms, guaranteeing in
this way the continuity of life. The physiology of heredity therefore
appears as the counterpart to those branches of the physiology of form
which deal with individual form and its restitutions. And our discussion
on heredity may be followed by our second appendix to this chief section
on form, an appendix regarding the outlines of systematics, evolution
and history.
Theoretical considerations on biology generally start, or at least,
used to start, from the evolution theory, discussing all other problems
of the physiology of form by the way only, as things of secondary
importance. You see from our programme, that we shall go just the
opposite way: evolution will come last of all, and will be treated
shortly; but the morphogenesis of the individual will be treated very
fully, and very carefully indeed.
Why then this deviation from what is the common practice? Because we do
not know very much about evolution at all, because in this field we are
just at the very beginning of what deserves the name of exact knowledge.
But concerning individual morphogenesis we really know, even at present,
if not very much, at least something, and that we know in a fairly exact
form, aided by the results of experiments.
And it will not be without its reward, if we restrict our aims in such a
manner, if we prefer to deal more fully with a series of problems, which
may seem at the first glance to be of less interest than others. After a
few lectures we shall find already that we may decide one very important
question about life merely by an analysis of individual form production,
and without any regard to problematic and doubtful parts of biology:
that we may decide the question, whether “life” is only a combination
of chemical and physical events, or whether it has its elemental laws,
laws of its own.
But to prepare the road that is to lead to such results we first have
to restrict our aims once more, and therefore the next lecture of this
course, which eventually is to touch almost every concept of philosophy
proper, will begin with the pure description of the individual
development of the common sea-urchin.
SECTION A
THE CHIEF RESULTS OF ANALYTICAL BIOLOGY
PART I
THE INDIVIDUAL ORGANISM WITH REGARD TO FORM AND METABOLISM
*A.* ELEMENTARY MORPHOGENESIS
EVOLUTIO AND EPIGENESIS IN THE OLD SENSE
The organism is a specific body, built up by a typical combination
of specific and different parts. It is implied in the words of this
definition, that the organism is different, not only from crystals, as
was mentioned in the last lecture, but also from all combinations of
crystals, such as those called dendrites and others, which consist of a
typical arrangement of identical units, the nature of their combination
depending on the forces of every single one of their parts. For this
reason dendrites, in spite of the typical features in their combination,
must be called aggregates; but the organism is not an aggregate even
from the most superficial point of view.
We have said before, what must have been familiar to you already, that
the organism is not always the same in its individual life, that it
has its development, leading from simpler to more complicated forms of
combination of parts; there is a “production of visible manifoldness”
carried out during development, to describe the chief character of that
process in the words of Wilhelm Roux. We leave it an open question in
our present merely descriptive analysis, whether there was already a
“manifoldness,” in an invisible state, before development, or whether
the phrase “production of manifoldness” is to be understood in an
absolute sense.
It has not always been granted in the history of biology, and of
embryology especially, that production of visible manifoldness is the
chief feature of what is called an organism’s embryology or ontogeny:
the eighteenth century is full of determined scientific battles over the
question. One school, with Albert von Haller and Bonnet as its leading
men, maintained the view that there was no production of different parts
at all in development, this process being a mere “evolutio,” that is,
a growth of parts already existing from the beginning, yes, from the
very beginning of life; whilst the other school, with C. F. Wolff and
Blumenbach at its head, supported the opposite doctrine of so-called
“epigenesis,” which has been proved to be the right one.
To some extent these differences of opinion were only the outcome of the
rather imperfect state of the optical instruments of that period. But
there were also deeper reasons beyond mere difficulties of description;
there were theoretical convictions underlying them. It is *impossible*,
said the one party, that there is any real production of new parts;
there *must* be such a production, said the other.
We ourselves shall have to deal with these questions of the theory of
organic development; but at present our object is narrower, and merely
descriptive. It certainly is of great importance to understand most
clearly that there actually *is* a “production of visible manifoldness”
during ontogenesis in the descriptive sense; the knowledge of the fact
of this process must be the very foundation of all studies on the theory
of development in any case, and therefore we shall devote this whole
lecture to studies in merely descriptive embryology.
But descriptive embryology, even if it is to serve merely as an
instance of the universality of the fact of epigenesis, can only be
studied successfully with reference to a concrete case. We select the
development of the common sea-urchin (*Echinus microtuberculatus*) as
such a case, and we are the more entitled to select this organism rather
than another, because most of the analytical experimental work, carried
out in the interests of a real theory of development, has been done
on the germs of this animal. Therefore, to know at least the outlines
of the individual embryology of the Echinus may indeed be called the
*conditio sine qua non* for a real understanding of what is to follow.
THE CELL[3]
[3] E. B. Wilson, *The Cell in Development and Inheritance*, New York,
Macmillan, 1896.
You are aware that all organisms consist of organs and that each of
their organs has a different function: the brain, the liver, the eyes,
the hands are types of organs in animals, as are the leaves and the
pistils in plants.
You are also aware that, except in the lowest organisms, the so-called
Protista, all organs are built up of cells. That is a simple fact of
observation, and I therefore cannot agree with the common habit of
giving to this plain fact the title of cell-“theory.” There is nothing
theoretical in it; and, on the other hand, all attempts to conceive
the organism as a mere aggregate of cells have proved to be wrong. It
is *the whole* that uses the cells, as we shall see later on, or that
may not use them: thus there is nothing like a “cell-theory,” even in a
deeper meaning of the word.
The cell may have the most different forms: take a cell of the skin, of
a muscle, of a gland, of the wood in plants as typical examples. But in
every case two parts may be distinguished in a cell: an outside part,
the protoplasm, and an inside part, the nucleus, to leave out of special
account several others, which, by the way, may only be protoplasmatic
modifications.
Protoplasm is a mere name for what is not the nucleus; in any case it is
not a homogeneous chemical compound; it consists of many such compounds
and has a sort of architecture; all organic functions are based upon
its metabolism. The nucleus has a very typical structure, which stands
in a close relation to its behaviour during the most characteristic
morphological period of the cell: during its division. Let us devote a
few words to a consideration of this division and the part the nucleus
plays in it; it will directly bear on future theoretical considerations
about development.
There is a certain substance in every nucleus of a cell which stains
most markedly, whenever cells are treated with pigments: the name
of “chromatin” has been given to it. The chromatin always gives the
reaction of an acid, while protoplasm is basic; besides that it seems to
be a centre of oxidation. Now, when a division of a cell is to occur,
the chromatin, which had been diffusely distributed before, in the form
of small grains, arranges itself into a long and very much twisted
thread. This thread breaks, as it were by sections, into almost equal
parts, typical in number for each species, and each of these parts is
split at full length. A certain number of pairs of small threads, the
so-called “chromosomes,” are the ultimate result of this process, which
intentionally has been described a little schematically, the breaking
and the splitting in fact going on simultaneously or occasionally even
in reverse order. While what we have described is performing in the
nucleus, there have happened some typical modifications in protoplasm,
and then, by an interaction of protoplasmatic and nuclear factors, the
first step in the actual division of the cell begins. Of each pair of
the small threads of chromatin one constituent is moved to one side of
the cell, one to the other; two daughter-nuclei are formed in this way;
the protoplasm itself at the same time forms a circular furrow between
them; the furrow gets deeper and deeper; at last it cuts the cell in
two, and the division of the cell is accomplished.
Not only is the growth of the already typically formed organism carried
out by a series of cell-divisions, but also development proper in our
sense, as a “production of visible manifoldness,” is realised to a great
extent by the aid of such divisions, which therefore may indeed be said
to be of very fundamental importance (Fig. 1).
[Illustration: Fig. 1.--Diagram of Cell-Division (*after* Boveri).
*a.* Resting cell; the chromatin distributed in the form of small
granules inside the nucleus. Outside the nucleus is the “centrosome,”
not mentioned in the text.
*b.* Beginning of division; the chromatin arranged in the form of a long
thread. Centrosome divided in two.
*c.* The thread of chromatin cut into four parts, the “chromosomes.”
*d.* The four parts of the chromatin arranged symmetrically between the
centrosomes and the star-like “spheres.”
*e.* Each of the chromosomes split at full length.
*f.* Beginning of division of protoplasm; the two parts of each
chromosome separated.
*g.* End of cell-division.]
Each cell-division which promotes growth is followed by the
enlargement of the two daughter-cells which result from it; these two
daughter-elements attain the exact size of the mother-cell before
division, and as soon as this size is reached a new division begins:
so the growth of the whole is in the main the result of the growth of
the elements. Cell-divisions during real organ-formation may behave
differently, as will be described at a proper occasion.
THE EGG: ITS MATURATION AND FERTILISATION
We know that all the organs of an animal or plant consist of cells, and
we know what acts a cell can perform. Now there is one very important
organ in all living beings, which is devoted to reproduction. This
organ, the so-called ovary in animals, is also built up of cells,
and its single cells are called the eggs; the eggs originated by
cell-division, and cell-division is to lead from them to the new adult.
But, with a very few exceptions, the egg in the ovary is not able to
accomplish its functions, unless certain typical events have occurred,
some of which are of a merely preparatory kind, whilst the others are
the actual stimulus for development.
The preparatory ones are generally known under the name of “maturation.”
The egg must be “mature,” in order that it may begin development, or
even that it may be stimulated to it. Maturation consists of a rather
complicated series of phenomena: later on we shall have occasion to
mention, at least shortly, what happens in the protoplasm during its
course; as to the nuclear changes during maturation it may be enough
for our purposes to say, that there occur certain processes among the
chromosomes, which lead to an extension of half of them in the form
of two very small cells, the “directive cells” or “directive or polar
bodies,” as they have been somewhat cautiously called.
The ripe or mature egg is capable of being fertilised.
Before turning to this important fact, which, by the way, will bring us
to our specially chosen type, the Echinus, a few words may be devoted to
the phenomenon of “parthenogenesis,” that is to say, the possibility
of development without fertilisation, since owing to the brilliant
discoveries of the American physiologist, Jacques Loeb, this topic forms
one of the centres of biological interest at present. It has long been
known that the eggs of certain bees, lice, crayfishes, and other animals
and also plants, are capable of development without fertilisation at
all. Now Richard Hertwig and T. H. Morgan already had shown, that at
least nuclear division may occur in the eggs of other forms--in the egg
of the sea-urchin for instance--when these eggs are exposed to some
chemical injuries. But Loeb[4] succeeded in obtaining a full development
by treating the eggs of echinoderms with chloride of magnesium; thus
artificial parthenogenesis had been discovered. Later researches have
shown that artificial parthenogenesis may occur in all classes of the
animal kingdom and may be provoked by all sorts of chemical or physical
means. We do not know at present in what the proper stimulus consists
that must be supposed here to take the place of fertilisation; it seems,
of course, highly probable that it is always the same in the last
resort.[5]
[4] *Amer. Journ. Physiol.* vols. iii. and iv. 1900.
[5] According to Delage (*Arch. Zool. exp.*, 3 sér. 10, 1902), it is
indifferent for the realisation of artificial parthenogenesis, whether
but one, or both, or neither of the “polar bodies” has been formed. But
the egg must be in the first stages of maturation to the extent that the
“nuclear membrane” must be already dissolved.
But enough about processes, which at present are of a highly scientific,
but hardly of any philosophic interest.
By fertilisation proper we understand the joining of the male element,
the spermatozoon or the spermia, with the female element, the egg. Like
the egg, the spermatozoon is but a cell, though the two differ very much
from one another in the relation between their protoplasm and nucleus:
in all eggs it is the protoplasm which is comparatively very large, if
held together with somatic cells, in the spermatozoon it is the nucleus.
A large amount of reserve material, destined for the growth of the
future being, is the chief cause of the size of the egg-protoplasm. The
egg is quite or almost devoid of the faculty of movement, while on the
contrary, movement is the most typical feature of the spermia. Its whole
organisation is adapted to movement in the most characteristic manner:
indeed, most spermatozoa resemble a swimming infusorium, of the type
of Flagellata, a so-called head and a moving tail are their two chief
constituents; the head is formed almost entirely of nuclear substance.
It seems that in most cases the spermatozoa swim around at random and
that their union with the eggs is assured only by their enormous number;
only in a few cases in plants have there been discovered special stimuli
of a chemical nature, which attract the spermia to the egg.
But we cannot enter here more fully into the physiology of
fertilisation, and shall only remark that its real significance is by no
means clear.[6]
[6] The older theories, attributing to fertilisation (or to
“conjugation,” *i.e.* its equivalent in Protozoa), some sort of
“renovation” or “rejuvenescence” of the race, have been almost
completely given up. (See Calkins, *Arch. für Entwickelungsmechanik*,
xv. 1902). R. Hertwig recently has advocated the view, that abnormal
relations between the amounts of nuclear and of protoplasmatic material
are rectified in some way by those processes. Teleologically, sexual
reproduction has been considered as a means of variability (Weismann),
but also as a means of preserving the type!
THE FIRST DEVELOPMENT PROCESS OF ECHINUS
Turning now definitively to the special kind of organism, chosen of our
type, the common sea-urchin, we properly begin with a few words about
the absolute size of its eggs and spermatozoa. All of you are familiar
with the eggs of birds and possibly of frogs; these are abnormally
large eggs, on account of the very high amount of reserve material they
contain. The almost spherical egg of our Echinus only measures about a
tenth of a millimetre in diameter; and the head of the spermatozoon has
a volume which is only the four-hundred-thousandth part of the volume
of the egg! The egg is about on the extreme limit of what can be seen
without optical instruments; it is visible as a small white point. But
the number of eggs produced by a single female is enormous and may
amount to hundreds of thousands; this is one of the properties which
render the eggs of Echinus so very suitable for experimental research;
you can obtain them whenever and in any quantity you like; and,
moreover, they happen to be very clear and transparent, even in later
stages, and to bear all kinds of operations well.
The spermia enters the egg, and it does so in the open water--another
of the experimental advantages of our type. Only one spermia enters
the egg in normal cases, and only its head goes in, the tail is left
outside. The moment that the head has penetrated the protoplasm of the
egg a thin membrane is formed by the latter. This membrane is very soft
at first, becoming much stronger later on; it is very important for all
experimental work, that by shaking the egg in the first minutes of its
existence the membrane can easily be destroyed without any damage to the
egg itself.
And now occurs the chief phenomenon of fertilisation: the nucleus of
the spermatozoon unites with the nucleus of the egg. When speaking of
maturation, we mentioned that half of the chromatin was thrown out of
the egg by that process: now this half is brought in again, but comes
from another individual.
It is from this phenomenon of nuclear union as the main character of
fertilisation that almost all theories of heredity assume their right to
regard the nuclei of the sexual cells as the true “seat” of inheritance.
Later on we shall have occasion to discuss this hypothesis from the
point of view of logic and fact.
After the complete union of what are called the male and the female
“pronuclei,” the egg begins its development; and this development, in
its first steps, is simply pure cell-division. We know already the chief
points of this process, and need only add to what has been described,
that in the whole first series of the cell-divisions of the egg, or,
to use the technical term, in the whole process of the “cleavage” or
“segmentation” of it, there is never any growth of the daughter-elements
after each division, such as we know to occur after all cell-divisions
of later embryological stages. So it happens, that during cleavage the
embryonic cells become smaller and smaller, until a certain limit is
reached; the sum of the volumes of all the cleavage cells together is
equal to the volume of the egg.
But our future studies will require a more thorough knowledge of the
cleavage of our Echinus; the experimental data we shall have to describe
later on could hardly be properly understood without such knowledge.
The first division plane, or, as we shall say, the first cleavage
plane, divides the eggs into equal parts; the second lies at right
angles to the first and again divides equally: we now have a ring of
four cells. The third cleavage plane stands at right angles to the
first two; it may be called an equatorial plane, if we compare the egg
with a globe; it also divides equally, and so we now find two rings,
each consisting of four cells, and one above the other. But now the
cell-divisions cease to be equal, at least in one part of the egg: the
next division, which leads from the eight- to the sixteen-cell stage of
cleavage, forms four rings, of four cells each, out of the two rings of
the eight-cell stage. Only in one half of the germ, in which we shall
call the upper one, or which we might call, in comparison with a globe,
the northern hemisphere, are cells of equal size to be found; in the
lower half of the egg four very small cells have been formed at one
“pole” of the whole germ. We call these cells the “micromeres,” that
is, the “small parts,” on the analogy of the term “blastomeres,” that
is, parts of the germ, which is applied to all the cleavage cells in
general. The place occupied by the micromeres is of great importance
to the germ as a whole: the first formation of real organs will start
from this point later on. It is sufficient thus fully to have studied
the cleavage of our Echinus up to this stage: the later cleavage stages
may be mentioned more shortly. All the following divisions are into
equal parts; there are no other micromeres formed, though, of course,
the cells derived from the micromeres of the sixteen-cell stage always
remain smaller than the rest. All the divisions are tangential; radial
cleavages never occur, and therefore the process of cleavage ends at
last in the formation of one layer of cells, which forms the surface
of a sphere; it is especially by the rounding-up of each blastomere,
after its individual appearance, that this real surface layer of cells
is formed, but, of course, the condition, that no radial divisions
occur, is the most important one in its formation. When 808 blastomeres
have come into existence the process of cleavage is finished; a sphere
with a wall of cells and an empty interior is the result. That only 808
cells are formed, and not, as might be expected, 1024, is due to the
fact that the micromeres divide less often than the other elements; but
speaking roughly, of course, we may say that there are ten steps of
cleavage-divisions in our form; 1024 being equal to 2^{10}.
We have learned that the first process of development, the cleavage, is
carried out by simple cell-division. A few cases are known, in which
cell-division during cleavage is accompanied by a specific migration of
parts of the protoplasm in the interior of the blastomeres, especially
in the first two or first four; but in almost all instances cleavage
is as simple a process of mere division as it is in our sea-urchin.
Now the second step in development, at least in our form, is a typical
histological performance: it gives a new histological feature to all
of the blastomeres: they acquire small cilia on their outer side and
with these cilia the young germ is able to swim about after it has left
its membrane. The germ may be called a “blastula” at this stage, as it
was first called by Haeckel, whose useful denominations of the first
embryonic stages may conveniently be applied, even if one does not agree
with most, or perhaps almost all, of his speculations (Fig. 2).
[Illustration: Fig. 2.--Early Development of Echinus, the Common
Sea-urchin.
*a.* Two cells.
*b.* Four cells.
*c.* Eight cells, arranged in two rings of four, above one another.
*d.* Sixteen cells, four “micromeres” formed at the “vegetative” pole.
*e.* Optical section of the “blastula,” a hollow sphere consisting of
about one thousand cells, each of them with a small cilium.]
It is important to notice that the formation of the “blastula” from the
last cleavage stage is certainly a process of organisation, and may also
be called a differentiation with regard to that stage. But there is in
the blastula no trace of one *part* of the germ becoming different with
respect to others of its parts. If development were to go on in this
direction alone, high organisatory complications might occur: but there
would always be only one sort of cells, arranged in a sphere; there
would be only one kind of what is called “tissue.”
But in fact development very soon loads to true differences of the
parts of the germ with respect to one another, and the next step of the
process will enable us to apply different denominations to the different
parts of the embryo.
At one pole of the swimming blastula, exactly at the point where the
descendants of the micromeres are situated, about fifty cells lose
contact with their neighbours and leave the surface of the globe, being
driven into the interior space of it. Not very much is known about the
exact manner in which these changes of cellular arrangement are carried
out, whether the cells are passively pressed by their neighbours, or
whether, perhaps, in a more active manner, they change their surface
conditions; therefore, as in most ontogenetic processes, the description
had best be made cautiously in fairly neutral or figurative words.
The cells which in the above manner have entered the interior of the
blastula are to be the foundation of important parts of the future
organism; they are to form its connective tissue, many of its muscles,
and the skeleton. “Mesenchyme,” *i.e.* “what has been infused into the
other parts,” is the technical name usually applied to these cells.
We now have to learn their definite arrangement. At first they lie
as a sort of heap inside the cell wall of the blastula, inside the
“blastoderm,” *i.e.* skin of the germ. But soon they move from one
another, to form a ring round the pole at which they entered, and on
this ring a process takes place which has a very important bearing upon
the whole type of the organisation of the germ. You will have noticed
that hitherto the germ with regard to its symmetry has been a monaxial
or radial formation; the cleavage stages and the blastula with its
mesenchyme were forms with two different poles, lying at the ends of one
single line, and round this line everything was arranged concentrically.
But now what is called “bilateral symmetry” is established; the
mesenchyme ring assumes a structure which can be symmetrically divided
only by one plane, but divided in such a way, that one-half of it is the
mirror image of the other. A figure shows best what has occurred, and
you will notice (Fig. 3) two masses of cells in this figure, which have
the forms of spherical triangles: it is in the midst of these triangles
that the skeleton of the larva *originates*. The germ had an upper and
a lower side before: it now has got an upper and lower, front and back,
*right and left* half; it now has acquired that symmetry of organisation
which our own body has; at least it has got it as far as its mesenchyme
is concerned.
[Illustration: Fig. 3.--Formation of Mesenchyme in Echinus.
*a.* Outlines of blastula, side-view; mesenchyme forms a heap of cells
at the “vegetative” pole.
*a*_1. Heap of mesenchyme-cells from above.
*b.* Mesenchyme-cells arranged in a ring round the vegetative pole.
*c.* Mesenchyme-cells arranged in a bilateral-symmetrical figure;
primordia of skeleton in the midst of two spherical triangles.]
We leave the mesenchyme for a while and study another kind of
organogenesis. At the very same pole of the germ where the mesenchyme
cells originated there is a long and narrow tube of cell growing in,
and this tube, getting longer and longer, after a few hours of growth
touches the opposite pole of the larva. The growth of this cellular tube
marks the beginning of the formation of the intestine, with all that
is to be derived from it. The larva now is no longer a blastula, but
receives the name of “gastrula” in Haeckel’s terminology; it is built
up of the three “germ-layers” in this stage. The remaining part of the
blastoderm is called “ectoderm,” or outer layer; the newly-formed tube,
“endoderm,” or inner layer; while the third layer is the “mesenchyme”
already known to us.
The endoderm itself is a radial structure at first, as was the whole
germ in a former stage, but soon its free end bends and moves against
one of the sides of the ectoderm, against that side of it where the two
triangles of the mesenchyme are to be found also. Thus the endoderm
has acquired bilateral symmetry just as the mesenchyme before, and
as in this stage the ectoderm also assumes a bilateral symmetry
in its form, corresponding with the symmetrical relations in the
endoderm and the mesenchyme, we now may call the whole of our larva a
bilateral-symmetrical organisation.
It cannot be our task to follow all the points of organogenesis of
Echinus in detail. It must suffice to state briefly that ere long a
second portion of the mesenchyme is formed in the larva, starting from
the free end of its intestine tube; that the formation of the so-called
“coelum” occurs by a sort of splitting off from this same original
organ; and that the intestine itself is divided into three parts of
different size and aspect by two circular sections.
But we must not, I think, dismiss the formation of the skeleton so
quickly. I told you already that the skeleton has its first origin in
the midst of the two triangular cell-masses of the mesenchyme; but what
are the steps before it attains its typical and complicated structure?
At the beginning a very small tetrahedron, consisting of carbonate of
calcium, is formed in each of the triangles; the four edges of the
tetrahedron are produced into thin rods, and by means of a different
organogenesis along each of these rods the typical formation of the
skeleton proceeds. But the manner in which it is carried out is very
strange and peculiar. About thirty of the mesenchyme cells are occupied
in the formation of skeleton substance on each side of the larva. They
wander through the interior space of the gastrula--which at this stage
is not filled with sea water but with a sort of gelatinous material--and
wander in such a manner that they always come to the right places, where
a part of the skeleton is to be formed; they form it by a process of
secretion, quite unknown in detail; one of them forms one part, one the
other, but what they form altogether, is one whole.
When the formation of the skeleton is accomplished, the typical larva
of our Echinus is built up; it is called the “pluteus” (Fig. 4). Though
it is far from being the perfect adult animal, it has an independent
life of its own; it feeds and moves about and does not go through any
important changes of form for weeks. But after a certain period of
this species of independent life as a “larva,” the changes of form it
undergoes again are most fundamental: it must be transformed into the
adult sea-urchin, as all of you know. There are hundreds and hundreds
of single operations of organogenesis to be accomplished before that
end is reached; and perhaps the strangest of all these operations is a
certain sort of growth, by which the symmetry of the animal, at least
in certain of its parts--not in all of them--is changed again from
bilateral to radial, just the opposite of what happened in the very
early stages.
[Illustration: Fig. 4.--Larval Development of Echinus.
*A.* The gastrula.
*B.* Later stage, bilateral-symmetrical. Intestine begins to divide
into three parts.
*C.* Pluteus larva. S = Skeleton. I = Intestine.]
But we cannot follow the embryology of our Echinus further here; and
indeed we are the less obliged to do so, since in all our experimental
work we shall have to deal with it only as far as to the pluteus larva.
It is impossible under ordinary conditions to rear the germs up to the
adult stages in captivity.
You now, I hope, will have a general idea at least of the processes
of which the individual development of an animal consists. Of course
the specific features leading from the egg to the adult are different
in each specific case, and, in order to make this point as clear as
possible, I shall now add to our description a few words about what may
be called a comparative descriptive embryology.
COMPARATIVE EMBRYOLOGY
Even the cleavage may present rather different aspects. There may be a
compact blastula, not one surrounded by only one layer of cells as in
Echinus; or bilaterality may be established as early as the cleavage
stage--as in many worms and in ascidians--and not so late as in Echinus.
The formation of the germ layers may go on in a different order and
under very different conditions: a rather close relative of our Echinus,
for instance, the starfish, forms first the endoderm and afterwards
the mesenchyme. In many cases there is no tube of cells forming the
“endoderm,” but a flat layer of cells is the first foundation of all the
intestinal organs: so it is in all birds and in the cuttlefish. And,
as all of you know, of course, there are very many animal forms which
have no proper “larval” stage: there is one in the frog, the well-known
“tadpole,” but the birds and mammals have no larvae; that is to say,
there is no special stage in the ontogeny of these forms which leads an
independent life for a certain time, as if it were a species by itself,
but all the ontogenetical stages are properly “embryonic”--the germ is
always an “embryo” until it becomes the perfect young organism. And you
also know that not all skeletons consist of carbonate of calcium, but,
that there are skeletons of silicates, as in Radiolaria, and of horny
substance, as in many sponges. And, indeed, if we were to glance at the
development of plants also, the differences would seem to us probably
so great that all the similarities would seem to disappear.
But there are similarities, nevertheless, in all development, and we
shall now proceed to examine what they are. As a matter of fact, it was
especially for their sake that we studied the ontogeny of a special
form in such detail; one always sees generalities better if one knows
the specific features of at least one case. What then are the features
of most general and far-reaching importance, which may be abstracted
from the individual history of our sea-urchin, checked always by the
teachings of other ontogenies, including those of plants?
THE FIRST STEPS OF ANALYTICAL MORPHOGENESIS
If we look back upon the long fight of the schools of embryologists in
the eighteenth century about the question whether individual development
was to be regarded as a real production of visible manifoldness or as
a simple growth of visibly pre-existing manifoldness, whether it was
“epigenesis” or “evolutio,” there can be no doubt, if we rely on all
the investigations of the last hundred and fifty years, that, taken in
the descriptive sense, the theory of epigenesis is right. Descriptively
speaking there *is* a production of visible manifoldness in the course
of embryology: that is our first and main result. Any one possessed of
an average microscope may any day convince himself personally that it is
true.
In fact, true epigenesis, in the descriptive sense of the term, does
exist. One thing is formed “after” the other; there is not a mere
“unfolding” of what existed already, though in a smaller form; there is
no “evolutio” in the old meaning of the word.
The word “evolution” in English usually serves to denote the theory of
descent, that is of a real relationship of all organisms. Of course we
are not thinking here of this modern and specifically English meaning of
the Latin word *evolutio*. In its ancient sense it means to a certain
degree just the opposite; it says that there is no formation of anything
new, no transformation, but simply growth, and this is promoted not for
the race but for the individual. Keeping well in mind these historical
differences in the meaning of the word “evolutio,” no mistakes, it seems
to me, can occur from its use. We now shall try to obtain a few more
particular results from our descriptive study of morphogenesis, which
are nevertheless of a general bearing, being real characteristics of
organic individual development, and which, though not calculated of
themselves to further the problem, will in any case serve to prepare for
a more profound study of it.
The totality of the line of morphogenetic facts can easily be resolved
into a great number of distinct processes. We propose to call these
“elementary morphogenetic processes”; the turning in of the endoderm
and its division into three typical parts are examples of them. If we
give the name “elementary organs” to the distinct parts of every stage
of ontogeny which are uniform in themselves and are each the result of
one elementary process in our sense, we are entitled to say that each
embryological stage consists of a certain number of elementary organs.
The mesenchyme ring, the coelum, the middle-intestine, are instances of
such organs. It is important to notice well that the word elementary is
always understood here with regard to visible morphogenesis proper and
does not apply to what may be called elementary in the physiological
sense. An elementary process in our sense is a very distinct act of
form-building, and an elementary organ is the result of every one of
such acts.
The elementary organs are typical with regard to their position and with
regard to their histological properties. In many cases they are of a
very clearly different histological type, as for instance, the cells of
the three so-called germ-layers; and in other cases, though apparently
almost identical histologically, they can be proved to be different
by their different power of resisting injuries or by other means. But
there are not as many different types of histological structure as there
are typically placed organs: on the contrary there are many elementary
organs of the same type in different typical parts of the organism, as
all of you know to be the case with nerves and muscles. It will not be
without importance for our future theory of development, carefully to
notice this fact, that specialisation in the *position* of embryonic
parts is more strict than in their histology.
But elementary organs are not only typical in position and histology,
they are typical also with regard to their form and their relative size.
It agrees with what has been said about histology being independent of
typical position, that there may be a number of organs in an embryonic
stage, all in their most typical positions, which though all possessing
the same histology, may have different forms or different sizes or
both: the single bones of the skeleton of vertebrates or of adult
echinoderms are the very best instances of this most important feature
of organogenesis. If we look back from elementary organs to elementary
processes, the specialisation of the size of those organs may also be
said to be the consequence of a typical duration of the elementary
morphogenetic process leading to them.[7]
[7] The phrase “*ceteris paribus*” has to be added of course, as the
duration of each single elementary morphogenetic process is liable to
vary with the temperature and many other conditions of the medium.
I hardly need to say, that the histology, form, and size of elementary
organs are equally an expression of their present or future
physiological function. At least they prepare for this function by a
specific sort of metabolism which sets in very early.
The whole sequence of individual morphogenesis has been divided by some
embryologists into two different periods; there is a first period,
during which the foundations of the organisation of the “type” are
laid down, and a second period, during which the histo-physiological
specifications are modelled out (von Baer, Götte, Roux). Such a
discrimination is certainly justified, if not taken too strictly; but
its practical application would encounter certain difficulties in many
larval forms, and also, of course, in all plants.
Our mention of plants leads us to the last of our analytical results. If
an animal germ proceeds in its development from a stage *d* to the stage
*g*, passing through *e* and *f*, we may say that the whole of *d* has
become the whole of *f*, but we cannot say that there is a certain part
of *f* which is *d*, we cannot say that *f* is *d* + *a*. But in plants
we can: the stage *f* is indeed equal to *a* + *b* + *c* + *d* + *e* +
*a* [Transcriber’s note: probable typo for *f*] in vegetable organisms;
all earlier stages are actually visible as parts of the last one. The
great embryologist, Carl Ernst von Baer, most clearly appreciated these
analytical differences between animal and vegetable morphogenesis. They
become a little less marked if we remember that plants, in a certain
respect, are not simple individuals but colonies, and that among the
corals, hydroids, bryozoa, and ascidia, we find analogies to plants in
the animal kingdom; but nevertheless the differences we have stated are
not extinguished by such reasoning. It seems almost wholly due to the
occurrence of so many foldings and bendings and migrations of cells and
complexes of cells in animal morphogenesis, that an earlier stage of
their development seems *lost* in the later one; those processes are
almost entirely wanting in plants, even if we study their very first
ontogenetic stages. If we say that almost all production of surfaces
goes on outside in plants, inside in animals, we shall have adequately
described the difference. And this feature again leads to the further
diversity between animals and plants which is best expressed by calling
the former “closed,” the latter “open” forms: animals reach a point
where they are finished, plants never are finished, at least in most
cases.
I hope you will allow that I have tried to draw from descriptive and
comparative embryology as many general analytical results as are
possibly to be obtained. It is not my fault if there are not any
more, nor is it my fault if the results reached are not of the most
satisfactory character. You may say that these results perhaps enable
you to see a little more clearly and markedly than before a few of the
characters of development, but that you have not really learnt anything
new. Your disappointment--my own disappointment--in our analysis is due
to the use of pure description and comparison as scientific methods.
THE LIMITS OF PURE DESCRIPTION IN SCIENCE
We have analysed our descriptions as far as we could, and now we must
confess that what we have found cannot be the last thing knowable
about individual morphogenesis. There must be something deeper to be
discovered: we only have been on the surface of the phenomena, we
now want to get to the very bottom of them. Why then occurs all that
folding, and bending, and histogenesis, and all the other processes we
have described? There must be something that drives them out, so to say.
There is a very famous dictum in the *Treatise on Mechanics* by the
late Gustav Kirchhoff, that it is the task of mechanics to describe
completely and in the most simple manner all the motions that occur in
nature. These words, which may appear problematic even in mechanics,
have had a really pernicious influence on biology. People were extremely
pleased with them. “‘Describing’--that is just what we always have
done,” they said; “now we see that we have done just what was right;
a famous physicist has told us so.” They did not see that Kirchhoff
had added the words “completely and in the most simple manner”; and
moreover, they did not consider that Kirchhoff never regarded it as the
ultimate aim of physics to describe thunderstorms or volcanic eruptions
or denudations; yet it only is with such “descriptions” that biological
descriptions of *given* bodies and processes are to be compared!
Physicists always have used both experiment and hypothetical
construction--Kirchhoff himself did so in the most gifted manner. With
these aids they have gone through the whole of the phenomena, and what
they found to be ultimate and truly elemental, that alone may they
be said to have “described”; but they have “explained” by the aid of
elementalities what proved to be not elemental in itself.[8]
[8] We shall not avoid in these lectures the word “explain”--so much
out of fashion nowadays. To “explain” means to subsume under known
concepts, or rules, or laws, or principles, whether the laws or concepts
themselves be “explained” or not. Explaining, therefore, is always
relative: what is elemental, of course, is only to be described, or
rather to be stated.
It is the *method* of the physicists--not their results--that
morphogenesis has to apply in order to make progress; and this method
we shall begin to apply in our next lectures. Physiology proper has
never been so short-sighted and self-satisfied as not to learn from
other sciences, from which indeed there was very much to be learned; but
morphology has: the bare describing and comparing of descriptions has
been its only aim for about forty years or more, and lines of descent of
a very problematic character were its only general results. It was not
seen that science had to begin, not with problematic events of the past,
but with what actually happens before our eyes.
But before saying any more about the exact rational and experimental
method in morphology, which indeed may be regarded as a new method,
since its prevalence in the eighteenth century had been really
forgotten, we first shall have to analyse shortly some general attempts
to understand morphogenesis by means of hypothetic construction
exclusively. Such attempts have become very important as points of
issue for really exact research, and, moreover, they deserve attention,
because they prove that their authors at least had not quite forgotten
that there were still other problems to be solved in morphology than
only phylogenetical ones.
*B.* EXPERIMENTAL AND THEORETICAL MORPHOGENESIS
1. THE FOUNDATIONS OF THE PHYSIOLOGY OF DEVELOPMENT. “EVOLUTIO” AND
“EPIGENESIS”
THE THEORY OF WEISMANN
Of all the purely hypothetic theories on morphogenesis that of August
Weismann[9] can claim to have had the greatest influence, and to
be at the same time the most logical and the most elaborated. The
“germ-plasma” theory of the German author is generally considered as
being a theory of heredity, and that is true inasmuch as problems of
inheritance proper have been the starting-point of all his hypothetic
speculations, and also form in some respect the most valuable part
of them. But, rightly understood, Weismann’s theory consists of two
independent parts, which relate to morphogenesis and to heredity
separately, and it is only the first which we shall have to take into
consideration at present; what is generally known as the doctrine of the
“continuity of the germ-plasm” will be discussed in a later chapter.
[9] *Das Keimplasma*, Jena, 1892.
Weismann assumes that a very complicated organised structure, below
the limits of visibility even with the highest optical powers, is the
foundation of all morphogenetic processes, in such a way that, whilst
part of this structure is handed over from generation to generation as
the basis of heredity, another part of it is disintegrated during the
individual development, and directs development by being disintegrated.
The expression, “part” of the structure, first calls for some
explanation. Weismann supposes several examples, several copies, as it
were, of his structure to be present in the germ cells, and it is to
these copies that the word “part” has been applied by us: at least one
copy has to be disintegrated during ontogeny.
The morphogenetic structure is assumed to be present in the nucleus
of the germ cells, and Weismann supposes the disintegration of his
hypothetic structure to be accomplished by nuclear division. By the
cleavage of the egg, the most *fundamental* parts of it are separated
one from the other. The word “fundamental” must be understood as
applying not to proper elements or complexes of elements of the
organisation, but to the chief relations of symmetry; the first
cleavage, for instance, may separate the right and the left part of
the structure, the second one its upper and lower parts, and after the
third or equatorial cleavage all the principal eighths of our minute
organisation are divided off: for the minute organisation, it must now
be added, had been supposed to be built up differently in the three
directions of space, just as the adult organism is. Weismann concedes it
to be absolutely unknown in what manner the proper relation between the
parts of the disintegrated fundamental morphogenetic structure and the
real processes of morphogenesis is realised; enough that there may be
imagined such a relation.
At the end of organogenesis the structure is assumed to have been
broken up into its elements, and these elements, which may be chemical
compounds, determine the fate of the single cells of the adult organism.
Here let us pause for a moment. There cannot be any doubt that
Weismann’s theory resembles to a very high degree the old “evolutio”
doctrines of the eighteenth century, except that it is a little less
crude. The chick itself is not supposed to be present in the hen’s egg
before development, and ontogeny is not regarded as a mere growth of
that chick in miniature, but what really is supposed to be present in
the egg is nevertheless a something that in all its parts corresponds
to all the parts of the chick, only under a somewhat different
aspect, while all the relations of the parts of the one correspond
to the relations of the parts of the other. Indeed, only on such an
hypothesis of a fairly fixed and rigid relation between the parts of
the morphogenetic structure could it be possible for the disintegration
of the structure to go on, not by parts of organisation, but by parts
of symmetry; which, indeed, is a very strange, but not an illogical,
feature of Weismann’s doctrine.
Weismann is absolutely convinced that there must be a theory of
“evolutio,” in the old sense of the word, to account for the ontogenetic
facts; that “epigenesis” has its place only in descriptive embryology,
where, indeed, as we know, manifoldness in the *visible* sense is
produced, but that epigenesis can never form the foundation of a real
morphogenetic *theory*: theoretically one pre-existing manifoldness is
transformed into the other. An epigenetic theory would lead right beyond
natural science, Weismann thinks, as in fact, all such theories, if
fully worked out, have carried their authors to vitalistic views. But
vitalism is regarded by him as dethroned for ever.
Under these circumstances we have a good right, it seems to me, to speak
of a *dogmatic* basis of Weismann’s theory of development.
But to complete the outlines of the theory itself: Weismann was
well aware that there were some grave difficulties attaching to his
statements: all the facts of so-called adventitious morphogenesis in
plants, of regeneration in animals, proved that the morphogenetic
organisation could not be fully disintegrated during ontogeny. But these
difficulties were not absolute: they could be overcome: indeed, Weismann
assumes, that in certain specific cases--and he regarded all cases of
restoration of a destroyed organisation as due to specific properties
of the subjects, originated by roundabout variations and natural
selection--that in specific cases, specific arrangements of minute parts
were formed during the process of disintegration, and were surrendered
to specific cells during development, from which regeneration or
adventitious budding could originate if required. “Plasma of reserve”
was the name bestowed on these hypothetic arrangements.
Almost independently another German author, Wilhelm Roux,[10] has
advocated a theoretical view of morphogenesis which very closely
resembles the hypothesis of Weismann. According to Roux a minute
ultimate structure is present in the nucleus of the germ and directs
development by being divided into its parts during the series of nuclear
divisions.
[10] *Die Bedeutung der Kernteilungsfiguren*, Leipzig, 1883.
But in spite of this similarity of the outset, we enter an altogether
different field of biological investigation on mentioning Roux’s name:
we are leaving hypothetic construction, at least in its absoluteness,
and are entering the realms of scientific experiment in morphology.
EXPERIMENTAL MORPHOLOGY
I have told you already in the last lecture that, while in the
eighteenth century individual morphogenesis had formed the centre of
biological interest and been studied experimentally in a thoroughly
adequate manner, that interest gradually diminished, until at last
the physiology of form as an exact separate science was almost wholly
forgotten. At least that was the state of affairs as regards zoological
biology; botanists, it must be granted, have never lost the historical
continuity to such a degree; botany has never ceased to be regarded
as one science and never was broken up into parts as zoology was.
Zoological physiology and zoological morphology indeed were for many
years in a relationship to one another not very much closer than the
relation between philology and chemistry.
There were always a few men, of course, who strove against the current.
The late Wilhelm His,[11] instance, described the embryology of the
chick in an original manner, in order to find out the mechanical
relations of embryonic parts, by which passive deformation, as an
integrating part of morphogenesis, might be induced. He also most
clearly stated the ultimate aim of embryology to be the mathematical
derivation of the adult form from the distribution of growth in the
germ. To Alexander Goette[12] we owe another set of analytical
considerations about ontogeny. Newport, as early as 1850, and in
later years Pflüger and Rauber, carried out experiments on the eggs
of the frog, which may truly be called anticipatory of what was to
follow. But it was Wilhelm Roux,[13] now professor of anatomy at
Halle, who entered the field with a thoroughly elaborated programme,
who knew not only how to state the problem analytically, but also
how to attack it, fully convinced of the importance of what he did.
“Entwickelungsmechanik,”--mechanics of development--he called the “new
branch of anatomical science” of which he tried to lay the foundations.
[11] *Unsere Körperform*, Leipzig, 1875.
[12] *Die Entwickelungsgeschichte der Unke*, Leipzig, 1875.
[13] *Gesammelte Abhandlungen*, Leipzig, 1895. Most important
theoretical papers:--*Zeitschr. Biolog.* 21, 1885; *Die
Entwickelungsmechanik der Organismen*, Wien, 1890; *Vorträge und
Aufsätze über Entwickelungsmechanik*, Heft i., Leipzig, 1905.
I cannot let this occasion pass without emphasising in the most decided
manner how highly in my opinion Roux’s services to the systematic
exploration of morphogenesis must be esteemed. I feel the more obliged
to do so, because later on I shall have to contradict not only many
of his positive statements but also most of his theoretical views. He
himself has lately given up much of what he most strongly advocated only
ten years ago. But Roux’s place in the history of biological science can
never be altered, let science take what path it will.
It is not the place here to develop the logic of experiment; least of
all is it necessary in the country of John Stuart Mill. All of you know
that experiment, by its method of isolating the single constituents
of complicated phenomena, is the principal aid in the discovery of
so-called causal relations. Let us try then to see what causal
relations Wilhelm Roux established with the aid of morphogenetic
experiment.
THE WORK OF WILHELM ROUX
We know already that an hypothesis about the foundation of individual
development was his starting-point. Like Weismann he supposed that
there exists a very complicated structure in the germ, and that nuclear
division leads to the disintegration of that structure. He next tried to
bring forward what might be called a number of indicia supporting his
view.
A close relation had been found to exist in many cases between the
direction of the first cleavage furrows of the germ and the direction
of the chief planes of symmetry in the adult: the first cleavage, for
instance, very often corresponds to the median plane, or stands at right
angles to it. And in other instances, such as have been worked out into
the doctrine of so-called “cell-lineages,” typical cleavage cells were
found to correspond to typical organs. Was not that a strong support
for a theory which regarded cellular division as the principal means
of differentiation? It is true, the close relations between cleavage
and symmetry did not exist in every case, but then there had always
happened some specific experimental disturbances, *e.g.* influences of
an abnormal direction of gravity on account of a turning over of the
egg, and it was easy to reconcile such cases with the generally accepted
theory on the assumption of what was called “anachronism” of cleavage.
But Roux was not satisfied with mere indicia, he wanted a proof, and
with this intention he carried out an experiment which has become
very celebrated.[14] With a hot needle he killed one of the first two
blastomeres of the frog’s egg after the full accomplishment of its first
cleavage, and then watched the development of the surviving cell. A
typical half-embryo was seen to emerge--an organism indeed, which was as
much a half as if a fully formed embryo of a certain stage had been cut
in two by a razor. It was especially in the anterior part of the embryo
that its “halfness” could most clearly be demonstrated.
[14] *Virchow’s Archiv.* 114, 1888.
That seemed to be a proof of Weismann’s and Roux’s theory of
development, a proof of the hypothesis that there is a very complicated
structure which promotes ontogeny by its disintegration, carried out
during the cell divisions of embryology by the aid of the process of
nuclear division, the so-called “karyokinesis.”
To the dispassionate observer it will appear, I suppose, that the
conclusions drawn by Roux from his experiment go a little beyond their
legitimate length. Certainly some sort of “evolutio” is proved by
rearing half the frog from half the egg. But is anything proved, is
there anything discovered at all about the nucleus? It was only on
account of the common opinion about the part it played in morphogenesis
that the nucleus had been taken into consideration.
Things soon became still more ambiguous.
THE EXPERIMENTS ON THE EGG OF THE SEA-URCHIN
Roux’s results were published for the first time in 1888; three years
later I tried to repeat his fundamental experiment on another subject
and by a somewhat different method. It was known from the cytological
researches of the brothers Hertwig and Boveri that the eggs of the
common sea-urchin (*Echinus microtuberculatus*) are able to stand well
all sorts of rough treatment, and that, in particular, when broken into
pieces by shaking, their fragments will survive and continue to segment.
I took advantage of these facts for my purposes. I shook the germs
rather violently during their two-cell stage, and in several instances I
succeeded in killing one of the blastomeres, while the other one was not
damaged, or in separating the two blastomeres from one another.[15]
[15] *Zeitschr. wiss. Zool.* 53, 1891.
Let us now follow the development of the isolated surviving cell. It
went through cleavage just as it would have done in contact with its
sister-cell, and there occurred cleavage stages which were just half
of the normal ones. The stage, for instance, which corresponded to the
normal sixteen-cell stage, and which, of course, in my subjects was
built up of eight elements only, showed two micromeres, two macromeres
and four cells of medium size, exactly as if a normal sixteen-cell stage
had been cut in two; and the form of the whole was that of a hemisphere.
So far there was no divergence from Roux’s results.
The development of our Echinus proceeds rather rapidly, the cleavage
being accomplished in about fifteen hours. I now noticed on the evening
of the first day of the experiment, when the half-germ was composed of
about two hundred elements, that the margin of the hemispherical germ
bent together a little, as if it were about to form a whole sphere
of smaller size, and, indeed, the next morning a *whole* diminutive
blastula was swimming about. I was so much convinced that I should get
Roux’s morphogenetical result in all its features that, even in spite of
this whole blastula, I now expected that the next morning would reveal
to me the half-organisation of my subject once more; the intestine, I
supposed, might come out quite on one side of it, as a half-tube, and
the mesenchyme ring might be a half one also.
But things turned out as they were bound to do and not as I had
expected; there was a typically *whole* gastrula on my dish the next
morning, differing only by its small size from a normal one; and this
*small but whole* gastrula was followed by a whole and typical small
pluteus-larva (Fig. 5).
[Illustration: Fig. 5.--Illustration of Experiments on Echinus.
*a*_1 and *b*_1. Normal gastrula and normal pluteus.
*a*_2 and *b*_2. “Half”-gastrula and “half”-pluteus, that *ought* to result
from one of the first two blastomeres, when isolated, according to the
theory of “evolutio.”
*a*_3 and *b*_3. The small *but whole* gastrula and pluteus that actually
*do* result.]
That was just the opposite of Roux’s result: one of the first two
blastomeres had undergone a half-cleavage as in his case, but then it
had become a whole organism by a simple process of rearrangement of its
material, without anything that resembled regeneration, in the sense of
a completion by budding from a wound.
If one blastomere of the two-cell stage was thus capable of performing
the morphogenetical process in its totality, it became, of course,
*impossible* to allow that nuclear division had separated any sort of
“germ-plasm” into two different halves, and not even the protoplasm of
the egg could be said to have been divided by the first cleavage furrow
into unequal parts, as the postulate of the strict theory of so-called
“evolutio” had been. This was a very important result, sufficient
alone to overthrow at once the theory of ontogenetical “evolutio,”
the “Mosaiktheorie” as it had been called--not by Roux himself, but
according to his views--in its exclusiveness.
After first widening the circle of my observations by showing that
one of the first four blastomeres is capable of performing a whole
organogenesis, and that three of the first four blastomeres together
result in an absolutely perfect organism, I went on to follow up
separately one of the two fundamental problems which had been suggested
by my first experiment: was there anything more to find out about
the importance or unimportance of the single *nuclear* divisions in
morphogenesis?[16]
[16] *Zeitschr. wiss. Zool.* 55, 1892.
By raising the temperature of the medium or by diluting the sea-water to
a certain degree it proved at first to be possible to alter in a rather
fundamental way the type of the cleavage-stages without any damage to
the resulting organism. There may be no micromeres at the sixteen-cell
stage, or they may appear as early as in the stage of eight cells;
no matter, the larva is bound to be typical. So it certainly is not
necessary for all the cleavages to occur just in their normal order.
But of greater importance for our purposes was what followed. I
succeeded in pressing the eggs of Echinus between two glass plates,
rather tightly, but without killing them; the eggs became deformed to
comparatively flat plates of a large diameter. Now in these eggs all
nuclear division occurred at right angles to the direction of pressure,
that is to say, in the direction of the plates, as long as the pressure
lasted; but the divisions began to occur at right angles to their former
direction, as soon as the pressure ceased. By letting the pressure be
at work for different times I therefore, of course, had it quite in my
power to obtain cleavage types just as I wanted to get them. If, for
instance, I kept the eggs under pressure until the eight-cell stage was
complete, I got a plate of eight cells one beside the other, instead of
two rings, of four cells each, one above the other, as in the normal
case; but the next cell division occurred at right angles to the former
ones, and a sixteen-cell stage, of two plates of eight cells each, one
above the other, was the result. If the pressure continued until the
sixteen-cell stage was reached, sixteen cells lay together in one plate,
and two plates of sixteen cells each, one above the other, were the
result of the next cleavage.
We are not, however, studying these things for cytological, but for
morphogenetical purposes, and for these the cleavage phenomenon itself
is less important than the organogenetic result of it: all our subjects
resulted in *absolutely normal* organisms. Now, it is clear, that the
spatial relations of the different nuclear divisions to each other are
anything but normal, in the eggs subjected to the pressure experiments;
that, so to say, every nucleus has got quite different neighbours if
compared with the “normal” case. If that makes no difference, then
there *cannot* exist any close relation between the single nuclear
divisions and organogenesis at all, and the conclusion we have drawn
more provisionally from the whole development of isolated blastomeres
has been extended and proved in the most perfect manner. There ought to
result a morphogenetic chaos according to the theory of real “evolutio”
carried out by nuclear division, if the positions of the single nuclei
were fundamentally changed with regard to one another (Fig. 6). But now
there resulted not chaos, but the normal organisation: therefore it was
disproved in the strictest way that nuclear divisions have any bearing
on the origin of organisation; at least as far as the divisions during
cleavage come into account.
[Illustration: Fig. 6.--Pressure-experiments on Echinus.
*a*_1 and *b*_1. Two normal cleavage stages, consisting of eight and
sixteen cells.
*a*_2 and *b*_2. Corresponding stages modified by exerting pressure
until the eight-cell stage was finished. See text.]
On the egg of the frog (O. Hertwig), and on the egg of annelids (E. B.
Wilson), my pressure experiments have been carried out with the same
result.[17]
[17] In the pressure experiments I had altered the relative position of
the nuclei *in origine*. In later years I succeeded in disturbing the
arrangement of the fully formed cells of the eight-cell stage, and in
getting normal larvæ in spite of that in many cases. But as this series
of experiments is not free from certain complications--which in part
will be understood later on (see page 73)--it must suffice here to have
mentioned them. (For further information see my paper in *Archiv. f.
Entwickelungsmechanik*, xiv., 1902, page 500.)
ON THE INTIMATE STRUCTURE OF THE PROTOPLASM OF THE GERM
Nuclear division, as we have seen, cannot be the basis of organogenesis,
and all we know about the whole development of isolated blastomeres
seems to show that there exists nothing responsible for differentiation
in the protoplasm either.
But would that be possible? It cannot appear possible on a more profound
consideration of the nature of morphogenesis, it seems to me: as the
untypical agents of the medium cannot be responsible in any way for
the origin of a form combination which is most typical and specific,
there must be somewhere in the egg itself a certain factor which is
responsible at least for the general orientation and symmetry of it.
Considerations of this kind led me, as early as 1893,[18] to urge the
hypothesis that there existed, that there *must* exist, a sort of
intimate structure in the egg, including polarity and bilaterality as
the chief features of its symmetry, a structure which belongs to every
smallest element of the egg, and which might be imagined by analogy
under the form of elementary magnets.[19] This hypothetic structure
could have its seat in the protoplasm only. In the egg of echinoderms it
would be capable of such a quick rearrangement after being disturbed,
that it could not be observed but only inferred logically; there might,
however, be cases in which its real discovery would be possible. Indeed
Roux’s frog-experiment seems to be a case where it is found to be at
work: at least it seems very probable to assume that Roux obtained half
of a frog’s embryo because the protoplasm of the isolated blastomere had
preserved the “halfness” of its intimate structure, and had not been
able to form a small whole out of it.
[18] *Mitteil. Neapel. 11, 1893.*
[19] But the elementary magnets would have to be bilateral!
Of course it was my principal object to verify this hypothesis, and
such verification became possible in a set of experiments which my
friend T. H. Morgan and myself carried out together,[20] in 1895, on
the eggs of ctenophores, a sort of pelagic animals, somewhat resembling
the jelly-fish, but of a rather different inner organisation. The
zoologist Chun had found even before Roux’s analytical studies, that
isolated blastomeres of the ctenophore egg behave like parts of the
whole and result in a half-organisation like the frog’s germ does. Chun
had not laid much stress on his discovery, which now, of course, from
the new points of view, became a very important one. We first repeated
Chun’s experiment and obtained his results, with the sole exception
that there was a tendency of the endoderm of the half-larva of Beroë
to become more than “half.” But that was not what we chiefly wanted to
study. We succeeded in cutting away a certain mass of the protoplasm
of the ctenophore egg just before it began to cleave, without damaging
its nuclear material in any way: in all cases, where the cut was
performed at the side, there resulted a certain type of larvae from
our experiments which showed exactly the same sort of defects as were
present in larvae developed from one of the first two blastomeres alone.
[20] *Arch. Entw. Mech.* 2, 1895.
The hypothesis of the morphogenetic importance of *protoplasm* had thus
been proved. In our experiments there was all of the nuclear material,
but there were defects on one side of the protoplasm of the egg; and the
defects in the adult were found to correspond to these defects in the
protoplasm.
And now O. Schultze and Morgan succeeded in performing some experiments
which directly proved the hypothesis of the part played by protoplasm
in the subject employed by Roux, *viz.*, the frog’s egg. The first of
these investigators managed to rear two whole frog embryos of small
size, if he slightly pressed the two-cell stage of that form between
two plates of glass and turned it over; and Morgan,[21] after having
killed one of the first two blastomeres, as was done in the original
experiment of Roux, was able to bring the surviving one to a half or
to a whole development according as it was undisturbed or turned.
There cannot be any doubt that in both of these cases, it is the
possibility of a rearrangement of protoplasm, offered by the turning
over, which allows the isolated blastomere to develop as a whole. The
regulation of the frog’s egg, with regard to its becoming whole, may be
called facultative, whilst the same regulation of the egg of Echinus
is obligatory. It is not without interest to note that the first two
blastomeres of the common newt, *i.e.* of a form which belongs to the
other class of Amphibia, after a separation of *any* kind, *always*
develop as wholes, their faculty of regulation being obligatory, like
that of Echinus.
[21] *Anat. Anz.* 10, 1895.
Whole or partial development may thus be dependent on the power of
regulation contained in the intimate polar-bilateral structure of the
protoplasm. Where this is so, the regulation and the differences in
development are both connected with the chief relations of symmetry.
The development becomes a half or a quarter of the normal because
there is only one-half or one-quarter of a certain structure present,
one-half or one-quarter with regard to the very wholeness of this
structure; the development is whole, in spite of disturbances, if
the intimate structure became whole first. We may describe the
“wholeness,” “halfness,” or “quarterness” of our hypothetic structure
in a mathematical way, by using three axes, at right angles to one
another, as the base of orientation. To each of these, *x*, *y*, and
*z*, a certain specific state with regard to the symmetrical relations
corresponds; thence it follows that, if there are wanting all those
parts of the intimate structure which are determined, say, by a negative
value of *y*, by minus *y*, then there is wanting half of the intimate
structure; and this halfness of the intimate structure is followed by
the halfness of organogenesis, the dependence of the latter on the
intimate structure being established. But if regulation has restored,
on a smaller scale, the whole of the arrangement according to all values
of *x*, *y* and *z*, development also can take place completely (Fig. 7).
[Illustration: Fig. 7.--Diagram illustrating the intimate Regulation of
Protoplasm from “Half” to “Whole.”
The large circle represents the original structure of the egg. In all
cases where cleavage-cells of the two-cell stage are isolated this
original structure is only present as “half” in the beginning, say
only on the right (+*y*) side. Development then becomes “half,” if the
intimate structure remains half; but it becomes “whole” (on a smaller
scale) if a new whole-structure (small circle!) is formed by regulatory
processes.]
I am quite aware that such a discussion is rather empty and purely
formal, nevertheless it is by no means without value, for it shows
most clearly the differences between what we have called the intimate
structure of germs, responsible only for the general symmetry of
themselves and of their isolated parts, and another sort of possible
structure of the egg-protoplasm which we now shall have to consider, and
which, at the first glance, seems to form a serious difficulty to our
statements, as far at least as they claim to be of general importance.
The study of this other sort of germinal structure at the same time will
lead us a step farther in our historical sketch of the first years of
“Entwickelungsmechanik” and will bring this sketch to its end.
ON SOME SPECIFICITIES OF ORGANISATION IN CERTAIN GERMS
It was known already about 1890, from the careful study of what has
been called “cell-lineage,” that in the eggs of several families of
the animal kingdom the origin of certain organs may be traced back to
individual cells of cleavage, having a typical histological character
of their own. In America especially such researches have been carried
out with the utmost minuteness, E. B. Wilson’s study of the cell-lineage
of the Annelid *Nereis* being the first of them. If it were true that
nuclear division is of no determining influence upon the ontogenetic
fate of the blastomeres, only peculiarities of the different parts of
the protoplasm could account for such relations of special cleavage
cells to special organs. I advocated this view as early as in 1894,
and it was proved two years later by Crampton, a pupil of Wilson’s,
in some very fine experiments performed on the germ of a certain
mollusc.[22] The egg of this form contains a special sort of protoplasm
near its vegetative pole, and this part of it is separated at each
of the first two segmentations by a sort of pseudo-cleavage, leading
to stages of three and five separated masses instead of two and four,
the supernumerary mass being the so-called “yolk-sac” and possessing
no nuclear elements (Fig. 8). Crampton removed this yolk-sac at the
two-cell stage, and he found that the cleavage of the germs thus
operated upon was normal except with regard to the size and histological
appearance of one cell, and that the larvae originating from these
germs were complete in every respect except in their mesenchyme, which
was wanting. A special part of the protoplasm of the egg had thus been
brought into relation with quite a special part of organisation, *and
that special part of the protoplasm contained no nucleus*.
[22] *Arch. Entw. Mech.* 3, 1896.
[Illustration: Fig. 8.--The Mollusc Dentalium (*after* E. B. Wilson).
*a.* The egg, consisting of three different kinds of protoplasmatic
material.
*b.* First cleavage-stage. There are two cells and one “pseudo-cell,”
the yolk-sac, which contains no nucleus. This was removed in Crampton’s
experiment.]
GENERAL RESULTS OF THE FIRST PERIOD OF “ENTWICKELUNGSMECHANIK”
This experiment of Crampton’s, afterwards confirmed by Wilson himself,
may be said to have closed the first period of the new science of
physiology of form, a period devoted almost exclusively to the problem
whether the theory of nuclear division or, in a wider sense, whether the
theory of a strict “evolutio” as the basis of organogenesis was true or
not.
It was shown, as we have seen, that the theory of the “qualitatively
unequal nuclear division” (“qualitativ-ungleiche Kernteilung” in German)
certainly was not true, and that there also was no strict “evolutio”
in protoplasm. Hence Weismann’s theory was clearly disproved. There
certainly is a good deal of real “epigenesis” in ontogeny, a good deal
of “production of manifoldness,” not only with regard to visibility but
in a more profound meaning. But some sort of pre-formation had also
been proved to exist, and this pre-formation, or, if you like, this
restricted evolution, was found to be of two different kinds. First an
intimate organisation of the protoplasm, spoken of as its polarity and
bilaterality, was discovered, and this had to be postulated for every
kind of germs, even when it was overshadowed by immediate obligatory
regulation after disturbances. Besides that there were cases in which
a real specificity of special parts of the germ existed, a relation of
these special parts to special organs: but this sort of specification
also was shown to belong to the protoplasm.
It follows from all we have mentioned about the organisation of
protoplasm and its bearing on morphogenesis, that the eggs of different
animals may behave rather differently, in this respect, and that
the eggs indeed may be classified according to the degree of their
organisation. Though we must leave a detailed discussion of these
topics to morphology proper, we yet shall try shortly to summarise
what has been ascertained about them in the different classes of the
animal kingdom. A full regulation of the *intimate* structure of
isolated blastomeres to a new whole, has been proved to exist in the
highest degree in the eggs of all echinoderms, medusae, nemertines,
Amphioxus, fishes, and in one class of the Amphibia (the *Urodela*);
it is facultative only among the other class of Amphibia, the *Anura*,
and seems to be only partly developed or to be wanting altogether among
ctenophora, ascidia, annelids, and mollusca. Peculiarities in the
organisation of *specific parts* of protoplasm have been proved to occur
in more cases than at first had been assumed; they exist even in the
echinoderm egg, as experiments of the last few years have shown; even
here a sort of specification exists at the vegetative pole of the egg,
though it is liable to a certain kind of regulation; the same is true in
medusae, nemertines, etc.; but among molluscs, ascidians, and annelids
no regulation about the specific organisation of the germ in cleavage
has been found in any case.
The differences in the degree of regulability of the intimate germinal
structure may easily be reduced to simple differences in the physical
consistency of their protoplasm.[23] But all differences in specific
organisation must remain as they are for the present; it will be one of
the aims of the future theory of development to trace these differences
also to a common source.
[23] It deserves notice in this connection, that in some cases the
protoplasm of parts of a germ has been found to be more regulable in
the earliest stages, when it is very fluid, than later, when it is more
stiff.
That such an endeavour will probably be not without success, is clear,
I should think, from the mere fact that differences with regard to
germinal specific pre-formation do not agree in any way with the
systematic position of the animals exhibiting them; for, strange as it
would be if there were two utterly different kinds of morphogenesis, it
would be still more strange if there were differences in morphogenesis
which were totally unconnected with systematic relationship: the
ctenophores behaving differently from the medusae, and Amphioxus
differently from ascidians.
SOME NEW RESULTS CONCERNING RESTITUTIONS
We now might close this chapter, which has chiefly dealt with the
disproof of a certain sort of ontogenetic theories, and therefore
has been almost negative in its character, did it not seem desirable
to add at least a few words about the later discoveries relating to
morphogenetic restorations of the adult. We have learnt that Weismann
created his concept of “reserve plasma” to account for what little
he knew about “restitutions”: that is, about the restoration of lost
parts: he only knew regeneration proper in animals and the formation of
adventitious buds in plants. It is common to both of these phenomena
that they take their origin from typically localised points of the body
in every case; each time they occur a certain well-defined part of the
body is charged with the restoration of the lost parts. To explain
such cases Weismann’s hypothesis was quite adequate, at least in a
logical sense. But at present, as we shall discuss more fully in another
chapter, we know of some very widespread forms of restitution, in which
what is to be done for a replacement of the lost is not entrusted to
*one* typical part of the body in every case, but in which the whole
of the morphogenetic action to be performed is transferred in its
*single* parts to the *single* parts of the body which is accomplishing
restoration: each of its parts has to take an individual share in the
process of restoration, effecting what is properly called a certain kind
of “re-differentiation” (“Umdifferenzierung”), and this share varies
according to the relative position of the part in each case. Later on
these statements will appear in more correct form than at present, and
then it will become clear that we are fully entitled to emphasise at the
end of our criticism of Weismann’s theory, that his hypothesis relating
to restorations can be no more true than his theory of development
proper was found to be.
And now we shall pass on to our positive work.
We shall try to sketch the outlines of what might properly be called an
*analytical theory of morphogenesis*; that is, to explain the sum of our
knowledge about organic form-production, gained by experiment and by
logical analysis, in the form of a real system, in which each part will
be, or at least will try to be, in its proper place and in relation with
every other part. Our analytical work will give us ample opportunity of
mentioning many important topics of so-called general physiology also,
irrespective of morphogenesis as such. But morphogenesis is always to
be the centre and starting-point of our analysis. As I myself approach
the subject as a zoologist, animal morphogenesis, as before, will be the
principal subject of what is to follow.
2. ANALYTICAL THEORY OF MORPHOGENESIS[24]
[24] Compare my *Analytische Theorie der organischen Entwickelung*,
Leipzig, 1894, and my reviews in *Ergebnisse der Anatomie und
Entwickelungsgeschichte*, vols. viii. xi. xiv., 1899-1905. A shorter
review is given in *Ergebnisse der Physiologie*, vol. v., 1906. The full
literature will be found in these reviews.
α. THE DISTRIBUTION OF MORPHOGENETIC POTENCIES
*Prospective Value and Prospective Potency*
Wilhelm Roux did not fail to see that the questions of the locality and
the time of all morphogenetic differentiations had to be solved first,
before any problem of causality proper could be attacked. From this
point of view he carried out his fundamental experiments.
It is only in terminology that we differ from his views, if we prefer
to call our introductory chapter an analysis of the distribution of
morphogenetic potencies. The result will be of course rather different
from what Roux expected it would be.
Let us begin by laying down two fundamental concepts. Suppose we have
here a definite embryo in a definite state of development, say a
blastula, or a gastrula, or some sort of larva, then we are entitled
to study any special element of any special elementary organ of this
germ with respect to what is actually to develop out of this very
element in the future actual course of this development, whether it be
undisturbed or disturbed in any way; it is, so to say, the actual, *the
real fate* of our element, that we take in account. I have proposed to
call this real fate of each embryonic part in this very definite line
of morphogenesis its *prospective value* (“prospective Bedeutung” in
German). The fundamental question of the first chapter of our analytical
theory of development may now be stated as follows: Is the prospective
value of each part of any state of the morphogenetic line constant,
*i.e.* is it unchangeable, can it be nothing but one; or is it variable,
may it change according to different circumstances?
We first introduce a second concept: the term *prospective potency*
(“prospective Potenz” in German) of each embryonic element. The term
“prospective morphogenetic potency” is to signify the *possible
fate* of each of those elements. With the aid of our two artificial
concepts we are now able to formulate our introductory question thus:
Is the prospective potency of each embryonic part fully given by
its prospective value in a certain definite case; is it, so to say,
identical with it, or does the prospective potency contain more than the
prospective value of an element in a certain case reveals?
We know already from our historical sketch that the latter is true: that
the actual fate of a part need not be identical with its possible fate,
at least in many cases; that the potency of the first four blastomeres
of the egg of the sea-urchin, for instance, has a far wider range than
is shown by what each of them actually performs in even this ontogeny.
There are more morphogenetic possibilities contained in each embryonic
part than are actually realised in a special morphogenetic case.
As the most important special morphogenetic case is, of course, the
so-called “normal” one, we can also express our formula in terms of
special reference to it: there are more morphogenetic possibilities in
each part than the observation of the normal development can reveal.
Thus we have at once justified the application of analytical experiment
to morphogenesis, and have stated its most important results.
As the introductory experiments about “Entwickelungsmechanik” have shown
already that the prospective potency of embryonic parts, at least in
certain cases, *can* exceed their prospective value--that, at least in
certain cases, it can be different from it--the concept of prospective
potency at the very beginning of our studies puts itself in the centre
of analytical interest, leaving to the concept of prospective value the
second place only. For that each embryonic part actually has a certain
prospective value, a specified actual fate in every single case of
ontogeny, is clear from itself and does not affirm more than the reality
of morphogenetic cases in general; but that the prospective value of the
elements may change, that there is a morphogenetic power in them, which
contains more than actuality; in other words, that the term “prospective
potency” has not only a logical but a factual interest: all these points
amount to a statement not only of the most fundamental introductory
results but also of the actual *problems* of the physiology of form.
If at each point of the germ something else *can* be formed than
actually is formed, why then does there happen in each case just what
happens and nothing else? In these words indeed we may state the chief
problem of our science, at least after the fundamental relation of
the superiority of prospective potency to prospective value has been
generally shown.
We consequently may shortly formulate our first problem as the question
of the distribution of the prospective morphogenetic potencies in the
germ. Now this general question involves a number of particular ones.
Up to what stage, if at all, is there an absolutely equal distribution
of the potencies over all the elements of the germ? When such an equal
distribution has ceased to exist at a certain stage, what are then the
relations between the parts of different potency? How, on the other
hand, does a newly arisen, more specialised sort of potency behave with
regard to the original general potency, and what about the distribution
of the more restricted potency?
I know very well that all such questions will seem to you a little
formal, and, so to say, academical at the outset. We shall not fail to
attach to them very concrete meanings.
*The Potencies of the Blastomeres*
At first we turn back to our experiments on the egg of the sea-urchin
as a type of the germ in the very earliest stages. We know already that
each of the first two, or each of the first four, or three of the first
four blastomeres together may produce a whole organism. We may add that
the swimming blastula, consisting of about one thousand cells, when cut
in two quite at random, in a plane coincident with, or at least passing
near, its polar axis, may form two fully developed organisms out of its
halves.[25] We may formulate this result in the words: the prospective
potency of the single cells of a blastula of Echinus is the same for
all of them; their prospective value is as far as possible from being
constant.
[25] If the plane of section passes near the equator of the germ, two
whole larvae may be formed also, but in the majority of cases the
“animal” half does not go beyond the blastula. The specific features of
the organisation of the protoplasm come into account here. See also page
65, note 1.
But we may say even a little more: what actually will happen in each of
the blastula cells in any special case of development experimentally
determined depends on the position of that cell in the whole, if the
“whole” is put into relation with any fixed system of co-ordinates; or
more shortly, “the prospective value of any blastula cell is a function
of its position in the whole.”
I know from former experience that this statement wants a few words of
explanation. The word “function” is employed here in the most general,
mathematical sense, simply to express that the prospective value,
the actual fate of a cell, will change, whenever its position in the
whole is different.[26] The “whole” may be related to any three axes
drawn through the normal undisturbed egg, on the hypothesis that there
exists a primary polarity and bilaterality of the germ; the axes which
determine this sort of symmetry may, of course, conveniently be taken as
co-ordinates; but that is not necessary.
[26] A change of the position of the cell is of course effected by each
variation of the direction of the cut, which is purely a matter of
chance.
*The Potencies of Elementary Organs in General*
Before dealing with other very young germs, I think it advisable to
describe first an experiment which is carried out at a later stage of
our well-known form. This experiment will easily lead to a few new
concepts, which we shall want later on, and will serve, on the other
hand, as a basis of explanation for some results, obtained from the
youngest germs of some other animal species, which otherwise would seem
to be rather irreconcilable with what our Echinus teaches us.
You know, from the second lecture, what a gastrula of our sea-urchin
is. If you bisect this gastrula, when it is completely formed, or
still better, if you bisect the gastrula of the starfish, either along
the axis or at right angles to it, you get complete little organisms
developed from the parts: the ectoderm is formed in the typical manner
in the parts, and so is the endoderm; everything is proportionate and
only smaller than in the normal case. So we have at once the important
results, that, as in the blastula, so in the ectoderm and in the
endoderm of our Echinus or of the starfish, the prospective potencies
are the same for every single element: both in the ectoderm and in
the endoderm the prospective value of each cell is a “function of its
position” (Fig. 9).
[Illustration: Fig. 9.--The Starfish, *Asterias*.
*a*^1. Normal gastrula; may be bisected along the main axis or at right
angles to it (see dotted lines).
*a*^2. Normal larva, “*Bipinnaria*.”
*b*^1. Small but whole gastrula that results by a process of regulation
from the parts of a bisected gastrula.
*b*^2. Small *but whole* “*Bipinnaria*,” developed out of *b*^1.]
But a further experiment has been made on our gastrula. If at the moment
when the material of the future intestine is most distinctly marked in
the blastoderm, but not yet grown into a tube, if at this moment the
upper half of the larva is separated from the lower by an equatorial
section, you will get a complete larva only from that part which bears
the “Anlage” of the endoderm, while the other half will proceed in
morphogenesis very well but will form only ectodermal organs. By another
sort of experiment, which we cannot fully explain here, it has been
shown that the endoderm if isolated is also only able to form such
organs as are normally derived from it.
And so we may summarise both our last results by saying: though
ectoderm and endoderm have their potencies equally distributed amongst
their respective cells, they possess different potencies compared one
with the other. And the same relation is found to hold for all cases of
what we call elementary organs: they are “equipotential,” as we may say,
in themselves, but of different potencies compared with each other.
*Explicit and Implicit Potencies: Primary and Secondary Potencies*
We shall first give to our concept of “prospective potency” a few words
of further analytical explanation with the help of our newly obtained
knowledge.
It is clear from what we have stated that the prospective potencies of
the ectoderm and of the endoderm, and we may add, of every elementary
organ in relation to every other, differ between themselves and also in
comparison with the blastoderm, from which they have originated. But the
diversity of the endoderm with respect to the ectoderm is not of the
same kind as its diversity in respect to the blastoderm. The potency
of the endoderm and that of the ectoderm are both specialised in their
typical manner, but compared with the potency of the blastoderm they
may be said not only to be specialised but also to be *restricted*: the
potency of the blastoderm embraces the whole, that of the so-called
germ-layer embraces only part of the whole; and this species of
restriction becomes clearer and clearer the further ontogeny advances:
at the end of it in the “ultimate elementary organs” there is no
prospective potency whatever.
A few new terms will serve to state a little more accurately what
happens. Of course, with regard to all morphogenesis which goes on
*immediately* from the blastoderm, the potency of the blastoderm is
restricted as much as are the potencies of the germ layers. We shall
call this sort of immediate potency *explicit*, and then we see at
once that, with regard to their explicit potencies, there are only
differences among the prospective potencies of the elementary organs;
but with respect to the *implicit* potency of any of these organs, that
is with respect to their potency as embracing the faculties of all their
derivations, there are also not only differences but true morphogenetic
restrictions lying at the very foundations of all embryology.
But now those of you who are familiar with morphogenetic facts will
object to me, that what we have stated about all sorts of restrictions
in ontogeny is not true, and you will censure me for having overlooked
regeneration, adventitious budding, and so on. To some extent the
criticism would be right, but I am not going to recant; I shall only
introduce another new concept. We are dealing only with *primary*
potencies in our present considerations, *i.e.* with potencies which
lie at the root of true embryology, not with those serving to regulate
disturbances of the organisation. It is true, we have in some way
disturbed the development of our sea-urchin’s egg in order to study
it; more than that, it would have been impossible to study it at all
without some sort of disturbance, without some sort of operation.
But, nevertheless, no potencies of what may properly be called the
*secondary* or restitutive type have been aroused by our operations;
nothing happened except on the usual lines of organogenesis. It is
true, some sort of regulation occurred, but that is included among the
factors of ontogeny proper.
We shall afterwards study more fully and from a more general point of
view this very important feature of “primary regulation” in its contrast
to “secondary regulation” phenomena. At present it must be enough to
say that in speaking of the restriction of the implicit potencies in
form-building we refer only to potencies of the primary type, which
contain within themselves some properties of a (primary) regulative
character.
*The Morphogenetic Function of Maturation in the Light of Recent
Discoveries*
Turning again to more concrete matters, we shall first try, with the
knowledge acquired of the potencies of the blastoderm and the so-called
germ layers of Echinus, to understand certain rather complicated
results which the experimental morphogenetic study of other animal
forms has taught us. We know from our historical sketch that there are
some very important aberrations from the type, to which the Echinus
germ belongs,[27] *i.e.* the type with an equal distribution of the
potencies over all the blastomeres. We know not only that in cases where
a regulation of the intimate structure of the protoplasm fails to occur
a partial development of isolated cells will take place, but that there
may even be a typical disposition of typical cells for the formation of
typical organs only, without any regulability.
[27] The reader will remember (see page 65, note 1), that even the germ
of Echinus is not quite equipotential along its main axis, but it is
equipotential in the strictest sense around this axis. The germs of
certain medusae seem to be equipotential in every respect, even in their
cleavage stages.
Let us first consider the last case, of which the egg of mollusca is a
good type: here there is no equal distribution of potencies whatever,
the cleavage-cells of this germ are a sort of real “mosaic” with regard
to their morphogenetic potentialities. Is this difference between the
germ of the echinoderms and the molluscs to remain where it is, and
not to be elucidated any further? Then there would be rather important
differences among the germs of different animals, at least with regard
to the degree of the specification of their cleavage cells, or if we
ascribe differences among the blastomeres to the organisation of the
fertilised egg ready for cleavage, there would be differences in the
morphogenetic organisation of the egg-protoplasm: some eggs would be
more typically specialised at the very beginning of morphogenesis than
others.
In the first years of the study of “Entwickelungsmechanik” I pointed out
that it must never be forgotten that the egg itself is the result of
organogenesis. If, therefore, there are real mosaic-like specifications
in some eggs at the beginning of cleavage, or during it, there may
perhaps have been an *earlier* stage in the individual history of
the egg which did not show such specifications of the morphogenetic
structure. Two American authors share the merit of having proved
this hypothesis. Conklin showed, several years ago, that certain
intracellular migrations and rearrangements of material do happen in
the first stages of ovogenesis in certain cases, but it is to E. B.
Wilson[28] that science owes a proper and definitive elucidation of the
whole subject. Wilson’s researches, pursued not only by descriptive
methods,[29] but also by means of analytical experiment, led him to the
highly important discovery that the eggs of several forms (nemertines,
molluscs), which after maturation show the mosaic type of specification
in their protoplasm to a more or less high degree, fail to show any
kind of specification in the distribution of their potencies before
maturation has occurred. In the mollusc egg a certain degree of
specification is shown already before maturation, but nothing to be
compared with what happens afterwards; in the egg of nemertines there is
no specification at all in the unripe egg.
[28] *Journ. Exp. Zool.* 1, 1904.
[29] Great caution must be taken in attributing any specific
morphogenetic part to differently coloured or constructed materials,
which may be observed in the egg-protoplasm in certain cases. They may
play such a part, but in other cases they certainly do not (see Lyon,
*Arch. Entw. Mech.* 23, 1907). The final decision always depends on
experiment.
Maturation thus becomes a part of ontogeny itself; it is not with
fertilisation that morphogenesis begins, there is a sort of ontogeny
anterior to fertilisation.
These words constitute a summary of Wilson’s researches. Taken together
with the general results obtained about the potencies of the blastula
and the gastrula of Echinus, they reduce what appeared to be differences
of degree or even of kind in the specification of the egg-protoplasm *to
mere differences in the time of the beginning of real morphogenesis*.
What occurs in some eggs, as in those of Echinus, at the time of the
definite formation of the germ layers, leading to a specification and
restriction of their prospective potencies, may happen very much earlier
in other eggs. But there exists in *every* sort of egg an *earliest*
stage, in which all parts of its protoplasm are equal as to their
prospectivity, and in which there are no potential diversities or
restrictions of any kind.
So much for differences in the *real material* organisation of the germ
and their bearing on inequipotentialities of the cleavage cells.
*The Intimate Structure of Protoplasm: Further Remarks*
Where a typical half- or quarter-development from isolated blastomeres
happens to occur, we know already that the impossibility of a regulation
of the *intimate polar-bilateral* structure may account for it. As this
impossibility of regulation probably rests on rather simple physical
conditions[30] it may properly be stated that equal distribution of
potencies is not wanting but is only overshadowed here. In this respect
there exists a logical difference of fundamental importance between
those cases of so-called “partial” or better, “fragmental” development
of isolated blastomeres in which a certain embryonic organ is wanting
on account of its specific morphogenetic material being absent, and
those cases in which the “fragmental” embryo lacks complete “halves” or
“quarters” with regard to general symmetry on account of the symmetry
of its intimate structure being irregularly disturbed. This logical
difference has not always received the attention which it undoubtedly
deserves. Our hypothetical intimate structure in itself is, of course,
also a result of factors concerned in ovogenesis. Only in one case do
we actually know anything about its origin: Roux has shown that in the
frog it is the accidental path of the fertilising spermatozoon in the
egg which, together with the polar axis, normally determines the plane
of bilateral symmetry; but this symmetry may be overcome and replaced
by another, if gravity is forced to act in an abnormal manner upon the
protoplasm; the latter showing parts of different specific gravity in
the eggs of all Amphibia.
[30] It seems that these physical conditions also--besides the real
specifications in the organisation of the egg--may be different before
and after maturation or (in other cases) fertilisation. (See Driesch,
*Archiv f. Entwickelungsmechanik*, 7, p. 98; and Brachet, *ibid.* 22, p.
325.)
*The Neutrality of the Concept of “Potency”*
Now we may close our rather long chapter on the distribution of
potencies in the germ; it has been made long, because it will prove to
be very important for further analytical discussion; and its importance,
in great measure, is due to its freedom from prepossessions. Indeed,
the concept of prospective potency does not prejudice anything; we
have said, it is true, that limitations of potencies may be due to
the presence of specific parts of organisation in some cases; that,
at least, they may be connected therewith; but we have not determined
at all what a prospective potency really is, what the term really is
to signify. It may seem that such a state of things gives an air of
emptiness to our discussions, that it leaves uncertain what is the most
important. But, I think, our way of argument, which tries to reach the
problems of greatest importance by degrees, though it may be slow, could
hardly be called wrong and misleading.
β. THE “MEANS” OF MORPHOGENESIS
We now proceed to an analysis of what may properly be called the *means*
of morphogenesis, the word “means” being preferable to the more usual
one “conditions” in this connection, as the latter would not cover the
whole field. It is in quite an unpretentious and merely descriptive
sense that the expression “means” should be understood at present; what
is usually called “conditions” is part of the morphogenetic means in our
sense.
β′. *The Internal Elementary Means of Morphogenesis*
We know that all morphogenesis, typical or atypical, primary or
secondary, goes on by one morphogenetic elementary process following the
other. Now the very foundation of these elementary processes themselves
lies in the elementary functions of the organism as far as they result
in the formation of stable visible products. Therefore the elementary
functions of the organism may properly be called the internal “means” of
morphogenesis.
Secretion and migration are among such functions; the former happening
by the aid of chemical change or by physical separation, the latter by
the aid of changes in surface tension. But hardly anything more concrete
has been made out about these or similar points at present.
We therefore make no claim to offer a complete system of the internal
elementary means of morphogenesis. We shall only select from the whole
a few topics of remarkable morphogenetic interest, and say a few words
about each.
But, first of all, let us observe that the elementary means of
morphogenesis are far from being morphogenesis themselves. The word
“means” itself implies as much. It would be possible to understand each
of these single acts in morphogenesis as well as anything, and yet to be
as far from understanding the whole as ever. All means of morphogenesis
are only to be considered as the most general frame of events within
which morphogenesis occurs.
*Some Remarks on the Importance of Surface Tension in
Morphogenesis.*--There are a few purely physical phenomena which have
a special importance in organic morphology, all of them connected
with capillarity or surface tension. Soap-lather is a very familiar
thing to all of you: you know that the soap-solution is arranged here
in very thin planes separated by spaces containing air: it was first
proved by Berthold[31] that the arrangement of cells in organic tissues
follows the same type as does the arrangement of the single bubbles of
a soap-lather, and Bütschli[32] added to this the discovery that the
minute structure of the protoplasm itself is that of a foam also. Of
course it is not one fluid and one gas which make up the constituents
of the structure in the organisms, as is the case in the well-known
inorganic foams, but two fluids, which do not mix with one another. One
general law holds for all arrangements of this kind: the so-called law
of least surfaces, expressed by the words that the sum of all surfaces
existing is a minimum; and it again is a consequence of this law, if
discussed mathematically, that four lines will always meet in one point
and three planes in one line. This feature, together with a certain law
about the relation of the angles meeting in one line to the size of the
bubbles, is realised most clearly in many structures of organic tissues,
and makes it highly probable, at least in some cases, that capillarity
is at work here. In other cases, as for instance in many plants, a kind
of outside pressure, the so-called tissue tension, may account for the
arrangement in surfaces *minimae areae*. Cleavage stages are perhaps
the very best type in which our physical law is expressed: and here
it may be said to have quite a simple application whenever all of the
blastomeres are of the same physical kind, whilst some complications
appear in germs with a specialised organisation and, therefore,
with differences in the protoplasm of their single blastomeres. In
such instances we may say that the physical law holds as far as the
conditions of the system permit, these conditions ordinarily consisting
in a sort of non-homogeneity of the surfaces.
[31] *Studien über Protoplasmamechanik*, Leipzig, 1886.
[32] *Unters. üb. mikroskopische Schäume und das Protoplasma*, Leipzig,
1892.
It seems, from the researches of Dreyer,[33] that the formation of
organic skeletons may also be governed by the physically conditioned
arrangement of protoplasmatic or cellular elements, and some phenomena
of migration and rearrangement among cleavage cells, as described by
Roux, probably also belong here.
[33] *Jena. Zeitschr.* 26, 1892.
But let us never forget that the laws of surface tension only give
us the most general type of an arrangement of elements in all these
cases, nothing else. A physical law never accounts for the Specific!
Capillarity gives us not the least clue to it. As the organic substance,
at least in many cases, is a fluid, it must of course follow the general
laws of hydrostatics and hydrodynamics, but life itself is as little
touched by its fluid-like or foam-like properties as it is by the fact
that living bodies have a certain weight and mass.
All indeed that has been described may be said to belong, in the
broadest meaning of the word, to what is called by Roux “correlation of
masses,” though this author originally intended to express by this term
only some sorts of passive pressure and deformation amongst embryonic
parts as discovered especially by His.
We must be cautious in admitting that any organic feature has been
explained, even in the most general way, by the action of physical
forces. What at first seems to be the result of mechanical pressure may
afterwards be found to be an active process of growth, and what at first
seems to be a full effect of capillarity among homogeneous elements may
afterwards be shown to depend on specialised metabolic conditions of the
surfaces as its principal cause.[34]
[34] According to Zur Strassen’s results the early embryology of
*Ascaris* proceeds almost exclusively by cellular surface-changes: the
most typical morphogenetic processes are carried out by the aid of this
“means.” As a whole, the embryology of *Ascaris* stands quite apart and
presents a great number of unsolved problems; unfortunately, the germ of
this form has not been accessible to experiment hitherto.
There are other physical phenomena too, which assist morphogenesis;
osmotic pressure for instance, which is also well known to operate in
many purely physiological processes. But all these processes are only
means of the organism, and can never do more than furnish the general
type of events. They do not constitute life; they are *used* by life;
let it remain an open question, for the present, how the phenomenon of
“life” is to be regarded in general.[35]
[35] Rhumbler has recently published a general survey of all attempts to
“explain” life, and morphogenesis in particular, in a physico-chemical
way (“Aus dem Lückengebiet zwischen organismischer und anorganismischer
Natur,” *Ergeb. Anat. u. Entw.-gesch.* 15, 1906). This *very
pessimistic* survey is the more valuable as it is written by a convinced
“mechanist.”
*On Growth*.--Among the internal morphogenetical means which are of
a so-called physiological character, that is, which nobody claims to
understand physically at present, there is in the first place *growth*,
which must be regarded as a very essential one.
Analytically we must carefully discriminate between the increase in the
size of the cavities of an organism by a passive extension of their
surfaces and the proper growth of the individual cells, which again
may be due either to mere extension or to real assimilation. Osmotic
pressure, of course, plays an important part both in the growth of the
body-cavities and in simple cellular extension. We repeat the caution
against believing too much to be explained by this phenomenon: it is the
organism which by the secretion of osmotic substances in the cavities or
the protoplasm of the cells prepares the ground for growth even of this
osmotic sort. The real cellular growth which proceeds on the basis of
assimilation cannot, of course, be accounted for by osmotic events, not
even in its most general type.
Ontogenetical growth generally sets in, both in animals and in plants,
after the chief lines of organisation are laid out; it is only the
formation of the definite histological structures which usually runs
parallel to it.
*On Cell-division.*--We have already said a good deal about the
importance of cell-division in ontogeny: it accompanies very many of the
processes of organisation in all living beings. But even then, there are
the Protozoa, in the morphogenesis of which it does not occur at all,
and there have also become known many cases of morphogenesis in higher
animals, mostly of the type of regulation, in which cellular division
is almost or wholly wanting. Therefore, cellular division cannot be
the true reason of differentiation, but is only a process, which
though necessary in some cases, cannot be essential to it. It must be
conceded, I believe, that the same conclusion can be drawn from all our
experiments on very young stages of the germ.
The investigations of the last few years have made it quite clear that
even in organisms with a high power of morphogenetic regulation it is
always the form of the whole, but not the individual cell, which is
subjected to the regulation processes. Starting from certain results
obtained by T. H. Morgan, I was able to show that in all the small but
whole larvae, reared from isolated blastomeres, the size of the cells
remains normal, only their number being reduced; and Boveri has shown
most clearly that it is always the size of the nucleus--more correctly,
the mass of the chromatin--which determines how large a cell of a
certain histological kind is to be. In this view, the cell appears even
more as a sort of material used by the organism as supplied, just as
workmen can build the most different buildings with stones of a given
size.
β″. *The External Means of Morphogenesis*
We now know what internal means of morphogenesis are, and so we may
glance at some of the most important “outer means” or “conditions” of
organisation.
Like the adult, the germ also requires a certain amount of heat, oxygen,
and, when it grows up in the sea, salinity in the medium. For the germ,
as for the adult, there exists not only a minimum but also a maximum
limit of all the necessary factors of the medium; the same factor which
at a certain intensity promotes development, disturbs it from a certain
other intensity upwards.
Within the limits of this minimum and this maximum of every outside
agent there generally is an increase in the rate of development
corresponding to the increase of intensity of the agent. The
acceleration of development by heat has been shown to follow the law of
the acceleration of chemical processes by a rise of temperature; that
seems to prove that certain chemical processes go on during the course
of morphogenesis.
Almost all that has been investigated of the part played by the external
conditions of development has little bearing on specific morphogenesis
proper, and therefore may be left out of account here: we must, however,
lay great stress on the general fact that there *is* a very close
dependence of morphogenesis on the outside factors, lest we should be
accused afterwards of having overlooked it.
Of course all “external” means or conditions of morphogenesis can
actually relate to morphogenetic processes only by becoming in some way
“internal,” but we unfortunately have no knowledge whatever how this
happens. We at present are only able to ascertain what must necessarily
be accomplished in the medium, in order that normal morphogenesis may go
on, and we can only suppose that there exist certain specific internal
general states, indispensable for organogenesis but inaccessible to
present modes of investigation.[36]
[36] Compare the analytical discussions of Klebs, to whom we owe a great
series of important discoveries in the field of morphogenetic “means”
in botany. (*Willkürliche Entwickelungsänderungen bei Pflanzen*, Jena,
1903; see also *Biol. Centralblatt*, vol. xxiv., 1904, and my reply to
Klebs, *ibid.* 23, 1903.)
*The Discoveries of Herbst.*--There are but few points in the doctrine
of the external means or conditions of organogenesis which have a
more special bearing on the specification of proper form, and which
therefore require to be described here a little more fully. All
these researches, which have been carried out almost exclusively by
Herbst,[37] relate to the effect of the chemical components of sea-water
upon the development of the sea-urchin. If we select the most important
of Herbst’s results, we must in the first place say a few words on
the part taken by lime or calcium, not only in establishing specific
features of form, but in rendering individual morphogenesis possible at
all. Herbst has found that in sea-water which is deprived of calcium the
cleavage cells and many tissue cells also completely lose contact with
each other: cleavage goes on quite well, but after each single division
the elements are separated; at the end of the process you find the 808
cells of the germ together at the bottom of the dish, all swimming about
like infusoria. There seems to be some influence of the calcium salts
upon the physical state of the surfaces of the blastomeres.
[37] *Arch. Entw. Mech.* 17, 1904.
It is not without interest to note that this discovery has an important
bearing on the technical side of all experiments dealing with the
isolation of blastomeres. Since the separation of the single cleavage
elements ceases as soon as the germs are brought back from the mixture
without lime into normal sea-water, it of course is possible to separate
them up to any stage which it is desired to study, and to keep them
together afterwards. Thus, if for instance you want to study the
development of isolated cells of the eight-cell stage, you will leave
the egg in the artificial mixture containing no calcium until the
third cleavage, which leads from the four- to the eight-cell stage, is
finished. The single eight cells brought back to normal sea-water at
this point will give you the eight embryos you want. All researches
upon the development of isolated blastomeres since the time of Herbst’s
discovery have been carried out by this method, and it would have been
quite impossible by the old method of shaking to pursue the study into
such minute detail as actually has been done. It may be added that
calcium, besides its cell-uniting action, is also of primary importance
in the formation of the skeleton.
Among all the other very numerous studies of Herbst we need only mention
that potassium is necessary for the typical growth of the intestine,
just as this element has been found necessary for normal growth in
plants, and that there must be the ion SO_4, or in other terms,
sulphur salts present in the water, in order that the germs may acquire
their pigments and their bilateral symmetry. This is indeed a very
important result, though it cannot be said to be properly understood. It
is a fact that in water without sulphates the larvae of Echinus retain
the radial symmetry they have had in the very earliest stages, and may
even preserve that symmetry on being brought back to normal sea-water if
they have spent about twenty-four hours in the artificial mixture.
We may now leave the subject of Herbst’s attempts to discover the
morphogenetic function of the single constituents of normal sea-water,
and may devote a few words to the other branch of his investigations,
those dealing with the morphogenetic effects of substances which are not
present in the water of the sea, but have been added to it artificially.
Here, among many other achievements, Herbst has made the most
important discovery that all salts of lithium effect radical changes
in development.[38] I cannot describe fully here how the so-called
“lithium larva” originates; let me only mention that its endoderm is
formed outside instead of inside, that it is far too large, that there
is a spherical mass between the ectodermal and the endodermal part of
the germ, that a radial symmetry is established in place of the normal
bilateralism, that no skeleton exists, and that the mesenchyme cells
are placed in a quite abnormal position. All these features, though
abnormal, are typical of the development in lithium. The larvae present
no really pathological appearance at all, and, therefore, it may indeed
be said that lithium salts are able to change fundamentally the whole
course of morphogenesis. It detracts nothing from the importance of
these discoveries that, at present, they stand quite isolated: only with
lithium salts has Herbst obtained such strange results, and only upon
the eggs of echinids, not even upon those of asterids, do lithium salts
act in this way.
[38] *Zeitschr. wiss. Zool.* 55, 1902; and *Mitt. Neapel.* 11, 1903.
γ. THE FORMATIVE CAUSES OR STIMULI
*The Definition of Cause*
We cannot begin the study of the “causes” of the differentiation of
form without a few words of explanation about the terminology which we
shall apply. Causality is the most disputed of all categories; many
modern scientists, particularly in physics, try to avoid the concept of
cause altogether, and to replace it by mere functional dependence in
the mathematical meaning of the term. They claim to express completely
by an equation all that is discoverable about any sort of phenomena
constantly connected.
I cannot convince myself that such a very restricted view is the right
one: it is very cautious, no doubt, but it is incomplete, for we *have*
the concept of the acting “cause” in our Ego and are *forced* to search
for applications of it in Nature. On the other hand, it does not at all
escape me that there are many difficulties, or rather ambiguities, in
applying it.
We may call the “cause” of any event, the sum total of all the
constellations of facts which must be completed in order that the event
may occur; it is in this meaning, for instance, that the first principle
of energetics applies the term in the words *causa aequat effectum*.
But, by using the word only in this very general sense, we deprive
ourselves of many conveniences in the further and more particular study
of Nature. Would it be better to say that the “cause” of any event is
the very last change which, after all the constellations necessary for
its start are accomplished, must still take place in order that the
event may actually occur? Let us see what would follow from such a use
of the word causality. We here have an animal germ in a certain stage,
say a larva of Echinus, which is just about to form the intestine; all
the internal conditions are fulfilled, and there is also a certain
temperature, a certain salinity, and so on, but there is no oxygen in
the water: the intestine; of course, will not grow in such a state of
things, but it soon will when oxygen is allowed to enter the dish.
Is, therefore, oxygen the cause of the formation of the intestine of
echinus? Nobody, I think, would care to say so. By such reasoning,
indeed, the temperature, or sodium, might be called the “cause” of
any special process of morphogenesis. It, therefore, seems to be of
little use to give the name of cause to that factor of any necessary
constellation of events which accidentally happens to be the last that
is realised. But what is to be done then?
Might we not say that the cause of any morphogenetic process is that
typical property, or quality, or change, on which its specific character
depends, on which depends for example, the fact that now it is the
intestine which appears, while at another time it is the lens of the
eye? We might very well, but we already have our term for this sort
of cause, which is nothing else than our prospective potency applied
to that elementary organ from which the new process takes its origin.
The prospective potency indeed is the truly immanent cause of every
specification affecting single organogenetic processes. But we want
something more than this.
We may find what we want by considering that each single elementary
process or development not only has its specification, but also has
its specific and typical place in the whole--its locality. Therefore
we shall call the “cause” of a single morphogenetic process, that
occurrence on which depends its *localisation*, whether its specific
character also partly depends on this “cause” or not.[39]
[39] In certain cases part of the specific feature of the process in
question may also depend on the “cause” which is localising it, *e.g.*
in the galls of plants.
This definition of “cause” in morphology may be artificial; in any
case it is clear. And at the same time the concepts of the prospective
potency and of the “means” of organogenesis now acquire a clear and
definite meaning: potency is the real basis of the specific character
of every act in morphogenesis, and “means,” including conditions, are
the sum of all external and internal general circumstances which must be
present in order that morphogenetic processes may go on, without being
responsible for their specificity or localisation.
It is implied in these definitions of cause and potency, that the former
almost always will be of that general type which usually is called a
stimulus or “Auslösung,” to use the untranslatable German word. There is
no quantitative correspondence between our “cause” and the morphogenetic
effect.
*Some Instances of Formative and Directive Stimuli*
Again it is to Herbst that we owe not only a very thorough logical
analysis of what he calls “formative and directive stimuli”[40] but also
some important discoveries on this subject. We cannot do more here than
barely mention some of the most characteristic facts.
[40] Herbst, “Ueber die Bedeutung die Reizphysiologie für die kausale
Auffassung von Vorgängen in der tierischen Ontogenese” (*Biol.
Centralblatt*, vols. xiv., 1894, and xv., 1895); *Formative Reize in der
tierischen Ontogenese*, Leipzig, 1901. These important papers must be
studied by every one who wishes to become familiar with the subject. The
present state of science is reviewed in my articles in the *Ergebnisse
der Anatomie und Entwickelungsgeschichte*, vols. xi. and xiv., 1902 and
1905.
Amongst plants it has long been known that the direction of light or of
gravity may determine where roots or branches or other morphogenetic
formations are to arise; in hydroids also we know that these factors
of the medium may be at work[41] as morphogenetic causes, though most
of the typical architecture of hydroid colonies certainly is due to
internal causes, as is also much of the organisation in plants.
[41] Compare the important papers by J. Loeb, *Untersuchungen zur
physiologischen Morphologie der Tiere*, Würzburg, 1891-2.
Light and gravity are external formative causes; beside that they are
merely “localisers.” But there also are some external formative stimuli,
on which depends not only the place of the effect, but also part of its
specification. The galls of plants are the most typical organogenetic
results of such stimuli. The potencies of the plant and the specific
kind of the stimulus equally contribute to their specification; for
several kinds of galls may originate on one sort of leaves.
Scarcely any exterior formative stimuli are responsible for animal
organisation; and one would hardly be wrong in saying that this
morphogenetic independence in animals is due to their comparatively
far-reaching functional independence of those external agents which
have any sort of direction. But many organogenetic relations are known
to exist between the single parts of animal germs, each of these parts
being in some respect external to every other; and, indeed, it might
have been expected already *a priori*, that such formative relations
between the parts of an animal embryo must exist, after all we have
learned about the chief lines of early embryology. If differentiation
does not go on after the scheme of Weismann, that is, if it is not
carried out by true “evolutio” from within, how could it be effected
except from without? Indeed, every embryonic part may in some respect be
a possible cause for morphogenetic events, which are to occur on every
other part: it is here that the very roots of epigenesis are to be found.
Heliotropism and geotropism are among the well-known physiological
functions of plants: the roots are seen to bend away from the light and
towards the ground; the branches behave just in the opposite way. It now
has been supposed by Herbst that such “directive stimuli” may also be
at work among the growing or wandering parts of the embryo, that their
growth or their migration may be determined by the typical character of
other parts, and that real morphogenetic characters can be the result of
some such relation; a sort of “chemotropism” or “chemotaxis” may be at
work here. Herbst himself has discussed theoretically several cases of
organogenesis in which the action of directive stimuli is very probable.
What has become actually known by experiment is not very much at
present: the mesenchyme cells of Echinus are directed in their migration
by specified places in the ectoderm, the pigment cells of the yolk-sac
of the fish fundulus are attracted by its blood vessels, and nerves
may be forced to turn into little tubes containing brain substance;
but of course only the first two instances have any bearing on typical
morphogenesis.
The first case of an “internal formative stimulus” in the proper sense,
that is, of one embryonic part causing another to appear, was discovered
by Herbst himself. The arms of the so-called pluteus of the sea-urchin
are in formative dependence on the skeleton--no skeleton, no arms; so
many skeleton primordia,[42] in abnormal cases, so many arms; abnormal
position of the skeleton, abnormal position of the arms: these three
experimental observations form the proof of this morphogenetic relation.
[42] I use the word “primordia” for the German “Anlage”; it is better
than the word “rudiment,” as the latter may also serve to signify
the very last stage of a certain formation that is disappearing
(phylogenetically).
It may be simple mechanical contact, or it may be some chemical
influence that really constitutes the “stimulus” in this case;
certainly, there exists a close and very specific relation of the
localisation of one part of the embryo to another. Things are much the
same in another case, which, after having been hypothetically stated
by Herbst on the basis of pathological data, was proved experimentally
by Spemann. The lens of the eye of certain Amphibia is formed of their
skin in response to a formative stimulus proceeding from the so-called
primary optic vesicle. If this vesicle fails to touch the skin, no lens
appears; and, on the other hand, the lens may appear in quite abnormal
parts of the skin if they come into contact with the optic vesicle after
transplantation.
But formative dependence of parts may also be of different types.
We owe to Herbst the important discovery that the eyes of crayfishes,
after being cut off, will be regenerated in the proper way, if the optic
ganglion is present, but that an antenna will arise in their place
if this ganglion has also been removed. There must in this case be
some unknown influence of the formative kind on which depends, if not
regeneration itself, at least its special character.
In other cases there seems to be an influence of the central nervous
system on the regenerative power in general. Amphibia, for instance,
are said to regenerate neither their legs (Wolff), nor their tail
(Godlewski), if the nervous communications have been disturbed. But
in other animals there is no such influence; and in yet others, as
for instance, in Planarians, it must seem doubtful at present whether
the morphogenetic influence of the nervous system upon processes of
restoration is more than indirect; the movements of the animal, which
become very much reduced by the extirpation of the ganglia, being one of
the main conditions of a good regeneration.
Of course, all we have said about the importance of special materials
in the ripe germ, as bearing on specifically localised organisations,
might be discussed again in our present chapter, and our intimate
polar-bilateral structure of germs may also be regarded as embracing
formative stimuli, at any rate as far as the actual poles of this
structure are concerned. This again would bring us to the problem of
so-called “polarity” in general, and to the “inversion” of polarity,
that is to a phenomenon well known in plants and in many hydroids and
worms, viz., that morphogenetic processes, especially of the type of
restitutions, occur differently, according as their point of origin
represents, so to speak, the positive or the negative, the terminal or
the basal end of an axis, but that under certain conditions the reverse
may also be the case. But a fuller discussion of these important facts
would lead us deeper and deeper into the science of morphogenesis
proper, without being of much use for our future considerations.
And so we may close this section[43] on formative stimuli or “causes”
of morphogenesis by shortly adding, more on account of its factual
than of its logical interest, that the phenomenon of the determination
of sex,[44] according to the latest researches, seems to depend on
cytological events occurring in the very earliest embryonic stages,
say even before ontogeny, and not on formative stimuli proper[45]: it
seems, indeed, as if the sexual products themselves would account for
the sex of the individual produced by them, particularly if there were
differences in their chromatin.[46]
[43] A full analysis of the subject would not only have to deal with
formative stimuli as inaugurating morphogenetic processes, but also with
those stimuli which terminate or stop the single acts of morphogenesis.
But little is actually known about this topic, and therefore the reader
must refer to my other publications. I will only say here, that the end
of each single morphogenetic act may either be determined at the very
beginning or occur as an actual stopping of a process which otherwise
would go on for ever and ever; in the first case some terminating
factors are included in the very nature of the morphogenetic act itself.
[44] A full account of the present state of the subject will be found in
Morgan’s *Experimental Zoology*, New York, 1907.
[45] But there certainly exist many formative relations between the real
sexual organs and the so-called secondary sexual characters. Herbst has
given a full analytical discussion of all that is known on this subject;
but the facts are much more complicated than is generally supposed, and
do not lend themselves therefore to short description. See also Foges,
*Pflüger’s Arch.* 93, 1902.
[46] It seems that in some cases (*Dinophilus*, certain Arthropods)
the sexual products are invariably determined as “arrenogennetic”
or as “thelygennetic” (Wilson, *Journ. Exp. Zool.* ii. and iii.
1905-6), whilst in others (Amphibia) the state of maturation or
“super”-maturation determines the sex of the future organism (R.
Hertwig, *Verh. D. Zool. Ges.* 1905-7).
δ. THE MORPHOGENETIC HARMONIES
Let us now turn again to considerations of a more abstract kind: we have
become acquainted with some morphogenetic interactions among the parts
of a developing embryo; and, indeed, we can be sure that there exist far
more of such interactions than we know at present.
But it is far from being true that the development of each embryonic
part depends on the existence or development of every other one.
On the contrary, it is a very important and fundamental feature
of organogenesis that it occurs in separate lines, that is to
say, in lines of processes which may start from a common root, but
which are absolutely independent of one another in their manner of
differentiation. Roux has coined the term “self-differentiation” to
denote this phenomenon, and we admit that this term may be conveniently
used for the purpose, if only it can be kept in mind that its sense is
always relative, and that it is also negative. Suppose a part, *A*,
shows the phenomenon of self-differentiation: this means that the
further development of *A* is not dependent on certain other parts,
*B*, *C*, and *D*; it does *not* mean at all that *A* has not been
formatively dependent on some other parts, *E* or *F* at the time of
its first appearance, nor does it imply that there might not be many
formative actions among the constituents of *A* itself.
We indeed are entitled to say that the ectoderm of Echinus shows
“self-differentiation” with regard to the endoderm; it acquires its
mouth, for instance, as has been shown by experiment, even in cases
where no intestine is present at all (Fig. 10); but ectoderm and
endoderm both are formatively dependent on the intimate and the material
organisation of the blastoderm. It further seems from the most recent
experiments that the nerves and the muscles of the vertebrates are
independent of each other in their differentiation, but that their fate
is probably determined by formative processes in the very earliest
stages of ontogeny.
[Illustration: Fig. 10.--Pluteus-larva of Sphaerechinus.
The Intestine (i) is developed outside instead of inside (by means of
raising the temperature); but the mouth (r) is formed in its normal
place. S = Skeleton.]
The phenomenon of self-differentiation, properly understood, now may
help to the discovery of one most general character of all development.
If the phenomenon of self-differentiation really occurs in ontogeny
in its most different aspects, and if, on the other hand, in spite
of this relative morphogenetic independence of embryonic parts, the
resulting organism is one whole in organisation and in function, some
sort of *harmony of constellation*, as it may properly be styled, must
be said to be one of the most fundamental characters of all production
of individual form. In establishing this harmony we do nothing more
than describe exactly what happens: the harmony is shown by the fact
that there is a whole organism at the end, in spite of the relative
independence of the single events leading to it.
But still another sort of harmony is revealed in morphogenesis, by an
analysis of the general conditions of the formative actions themselves.
In order that these actions may go on properly the possibility must be
guaranteed that the formative causes may always find something upon
which to act, and that those parts which contain the potencies for the
next ontogenetic stage may properly receive the stimuli awaking these
potencies: otherwise there would be no typical production of form at
all. This, the second species of harmonious relations to be described in
ontogeny, may be called *causal harmony*; the term simply expresses the
unfailing relative condition of formative causes and cause-recipients.
Finally, in *functional harmony* we have an expression descriptive
of the unity of organic function, and so we may state, as the latest
result of our analytical theory of development up to this point, that
individual morphogenesis is marked by a *threefold harmony* among its
parts.
ε. ON RESTITUTIONS[47]
[47] Driesch, *Die organischen Regulationen*, Leipzig, 1901; Morgan,
*Regeneration*, New York, 1901.
At this stage we leave for a while our analytical studies of ontogeny
proper. We must not forget that typical ontogenesis is not the only form
in which morphogenesis can occur: the organic form is able to restore
disturbances of its organisation, and it certainly is to be regarded as
one of the chief problems of analytical morphogenesis to discover the
specific and real stimulus which calls forth the restoring processes.
For simply to say that the disturbance is the cause of the restoration
would be to evade the problem instead of attacking it. But there are
still some other problems peculiar to the doctrine of restitutions.
*A few Remarks on Secondary Potencies and on Secondary Morphogenetic
Regulations in General*
We have only briefly mentioned in a previous chapter that there
exist many kinds of potencies of what we call the secondary or truly
restitutive type, and that their distribution may be most various and
quite independent of all the potencies for the primary processes
of ontogeny proper. Let us first add a few words about the concept
of “secondary restitution” and about the distribution of secondary
potencies in general.
Primary ontogenetic processes founded upon primary potencies may *imply*
regulation, or more correctly, restitution in many cases: so it is,
when fragments of the blastula form the whole organism, or when the
mesenchyme cells of Echinus reach their normal final position by an
attraction on the part of specific localities of the ectoderm in spite
of a very abnormal original position enforced upon them by experiment.
In these cases we speak of primary regulations or restitutions;
disturbances are neutralised by the very nature of the process in
question. We speak of secondary restitution whenever a disturbance
of organisation is rectified by processes foreign to the realm of
normality; and these abnormal lines of events are revealed to us in the
first place by the activity of potencies which remain latent in ontogeny
proper.
We know already that a certain kind of secondary restitution has
been discovered lately, very contradictory to the theoretical views
of Weismann; the process of restoration being carried out not by any
definite part of the disturbed organisation, but by all the single
elements of it. The problem of the distribution of secondary potencies
in these cases of so-called “re-differentiation” is to form our special
study in the next chapter. In all other cases restoration processes
start from specific localities; if they occur on the site of the
wound which caused the disturbance, we speak of regeneration; if they
occur at some distance from the wound, we call them adventitious
processes. Besides these three types of processes of restitution there
may be mentioned a fourth one, consisting in what is generally called
compensatory hypertrophy; the most simple case of such a compensatory
process is when one of a pair of organs, say a kidney, becomes larger
after the other has been removed.[48] Finally, at least in plants, a
change of the directive irritability, of so-called “geotropism” for
instance, in certain parts may serve to restore other more important
parts.
[48] But real compensatory differentiation occurs in the cases of
so-called “hypertypy” as first discovered by Przibram and afterwards
studied by Zeleny: here the two organs of a pair show a different degree
of differentiation. Whenever the more specialised organ is removed the
less developed one assumes its form. Similar cases, which might simply
be called “compensatory heterotypy,” are known in plants, though only
relating to the actual fate of undifferentiated “Anlagen” in these
organisms. A leaf may be formed out of the Anlage of a scale, if all the
leaves are cut off, and so on.
In two of these general types of restitution, in regeneration proper and
in the production of adventitious organs, the potencies which underlie
these processes may be said to be “complex.” It is a complicated series
of events, a proper morphogenesis in itself, for which the potency
has to account, if, for instance, a worm newly forms its head by
regeneration, or if a plant restores a whole branch in the form of an
adventitious bud.
Such generalisations as are possible about the distribution of complex
potencies are reserved for a special part of our future discussion.
Secondary restitution is always, like ontogeny, a process of
morphogenesis, and therefore all the questions about single formative
stimuli, and about internal and external conditions or means, occur
again. But of course we cannot enter into these problems a second time,
and may only say that, especially in regeneration proper, the specific
type of the regenerative formation of any part may differ very much from
the ontogenetic type of its origin: the end of both is the same, but the
way can be even fundamentally different in every respect.
*The Stimuli of Restitutions*[49]
[49] For a fuller analysis compare my opening address delivered before
the section of “Experimental Zoology” at the Seventh Zoological
Congress, Boston, 1907: “The Stimuli of Restitutions” (see Proceedings
of that Congress).
But now we turn to the important question: what is the precise
stimulus[50] that calls forth processes of restitution; or, in other
words, what must have happened in order that restitution may occur?
[50] The problem of the stimulus of a secondary restitution as a
whole must not be confused with the very different question, what the
single “formative stimuli” concerned in the performance of a certain
restitutive act may be. With regard to restitution as a *whole* these
single “formative stimuli” might properly be said to belong to its
“internal means”--in the widest sense of the word.
That the operation in itself, by its removing of mechanical obstacles,
cannot be the true stimulus of any restitutions, is simply shown by all
those restitutions that do not happen at the place of the wound. If we
took a narrower point of view, and if we only considered regeneration
proper from the wound itself, we might probably at first be inclined to
advocate the doctrine that the removing of some obstacles might in fact
be the stimulus to the process of restoration; but, even then, why is
it that just what is wanted grows out? Why is there not only growth,
but specific growth, growth followed by specification? The removing
of an obstacle could hardly account for that. But, of course, taking
account of all the adventitious restitutions--that is, all restorations
not beginning at the wound itself--the theory that the removing of
obstacles is the stimulus to restoration becomes, as we have said, quite
impossible.[51]
[51] T. H. Morgan is very right in stating that, in regeneration, the
“obstacle” itself is newly formed by the mere process of healing,
previous to all restitution, and that true restitution happens all the
same.
But where then is the stimulus to be found? There is another rather
simple theory of the “Auslösung” of restitutions,[52] which starts
from the phenomena of compensatory hypertrophy and some occurrences
among plants. The removal of some parts of the organism, it is said,
will bring its other parts into better conditions of nutrition, and
therefore these parts, particularly if they are of the same kind, will
become larger. Granted for the moment that such a view may hold in cases
when one of a pair of glands becomes larger after the other has been
removed, or when pruning of almost all the leaves of a tree leads to the
rest becoming larger, it certainly must fail to explain the fact that
in other cases true *new* formations may arise in order to restore a
damaged part, or that the latter may be regenerated in its proper way.
For *merely quantitative* differences in the mixture of the blood or of
the nourishing sap in plants can never be a sufficient reason for the
highly typical and *qualitative* structure of newly-formed restitutions.
And even in the most simple cases of a mere increase in the size of
some parts, that is, in the simplest cases of so-called compensatory
hypertrophy,[53] it is at least doubtful, if not very improbable, that
the compensation is accomplished in such a purely passive way, because
we know that in other cases it is usually the growth of the young parts
that actively a
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