Transcriber’s notes:

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The rare spelling typos noted in the original text have been corrected 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. index entries with missing commas). The abbreviation viz. appears in both roman and italic font.

Formatting of entries in the Table of Contents does not accurately match that of the corresponding headings in the text, particularly the 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In Figure 12 caption, multiple ditto marks have been replaced by the relevant text for greater clarity.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

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tried to improve my previous books by adding notes or altering the arrangement; I also had left a good deal of things unpublished, and thus I often hoped that I might have occasion to arrange for a new, improved, and enlarged edition of those books.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

SECTION A.—THE CHIEF RESULTS OF ANALYTICAL BIOLOGY

PART I.—THE INDIVIDUAL ORGANISM WITH REGARD TO
FORM AND METABOLISM

PART II.—SYSTEMATICS AND HISTORY

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]

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.

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]

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).

Fig. 1.—Diagram of Cell-Division (after Boveri).

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]

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]

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 2¹⁰.

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).

Fig. 2.—Early Development of Echinus, the Common Sea-urchin.

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.

Fig. 3.—Formation of Mesenchyme in Echinus.

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.

Fig. 4.—Larval Development of Echinus.

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]

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 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]

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.

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.

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] for 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.

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.

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]

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).

Fig. 5.—Illustration of Experiments on Echinus.

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]

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.

Fig. 6.—Pressure-experiments on Echinus.

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]

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.

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.

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.

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).

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.

Fig. 8.—The Mollusc Dentalium (after E. B. Wilson).

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.

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]

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.

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.

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).

Fig. 9.—The Starfish, Asterias.