Chapter V: Part I: The Individual Organism with Regard to Form and Metabolism (2)

A close relation had been found to exist in many cases between the direction of the first cleavage furrows of the germ and the direction of the chief planes of symmetry in the adult: the first cleavage, for instance, very often corresponds to the median plane, or stands at right angles to it. And in other instances, such as have been worked out into the doctrine of so-called “cell-lineages,” typical cleavage cells were found to correspond to typical organs. Was not that a strong support for a theory which regarded cellular division as the principal means of differentiation? It is true, the close relations between cleavage and symmetry did not exist in every case, but then there had always happened some specific experimental disturbances, *e.g.* influences of an abnormal direction of gravity on account of a turning over of the egg, and it was easy to reconcile such cases with the generally accepted theory on the assumption of what was called “anachronism” of cleavage.

But Roux was not satisfied with mere indicia, he wanted a proof, and with this intention he carried out an experiment which has become very celebrated.[14] With a hot needle he killed one of the first two blastomeres of the frog’s egg after the full accomplishment of its first cleavage, and then watched the development of the surviving cell. A typical half-embryo was seen to emerge--an organism indeed, which was as much a half as if a fully formed embryo of a certain stage had been cut in two by a razor. It was especially in the anterior part of the embryo that its “halfness” could most clearly be demonstrated.

[14] *Virchow’s Archiv.* 114, 1888.

That seemed to be a proof of Weismann’s and Roux’s theory of development, a proof of the hypothesis that there is a very complicated structure which promotes ontogeny by its disintegration, carried out during the cell divisions of embryology by the aid of the process of nuclear division, the so-called “karyokinesis.”

To the dispassionate observer it will appear, I suppose, that the conclusions drawn by Roux from his experiment go a little beyond their legitimate length. Certainly some sort of “evolutio” is proved by rearing half the frog from half the egg. But is anything proved, is there anything discovered at all about the nucleus? It was only on account of the common opinion about the part it played in morphogenesis that the nucleus had been taken into consideration.

Things soon became still more ambiguous.

THE EXPERIMENTS ON THE EGG OF THE SEA-URCHIN

Roux’s results were published for the first time in 1888; three years later I tried to repeat his fundamental experiment on another subject and by a somewhat different method. It was known from the cytological researches of the brothers Hertwig and Boveri that the eggs of the common sea-urchin (*Echinus microtuberculatus*) are able to stand well all sorts of rough treatment, and that, in particular, when broken into pieces by shaking, their fragments will survive and continue to segment. I took advantage of these facts for my purposes. I shook the germs rather violently during their two-cell stage, and in several instances I succeeded in killing one of the blastomeres, while the other one was not damaged, or in separating the two blastomeres from one another.[15]

[15] *Zeitschr. wiss. Zool.* 53, 1891.

Let us now follow the development of the isolated surviving cell. It went through cleavage just as it would have done in contact with its sister-cell, and there occurred cleavage stages which were just half of the normal ones. The stage, for instance, which corresponded to the normal sixteen-cell stage, and which, of course, in my subjects was built up of eight elements only, showed two micromeres, two macromeres and four cells of medium size, exactly as if a normal sixteen-cell stage had been cut in two; and the form of the whole was that of a hemisphere. So far there was no divergence from Roux’s results.

The development of our Echinus proceeds rather rapidly, the cleavage being accomplished in about fifteen hours. I now noticed on the evening of the first day of the experiment, when the half-germ was composed of about two hundred elements, that the margin of the hemispherical germ bent together a little, as if it were about to form a whole sphere of smaller size, and, indeed, the next morning a *whole* diminutive blastula was swimming about. I was so much convinced that I should get Roux’s morphogenetical result in all its features that, even in spite of this whole blastula, I now expected that the next morning would reveal to me the half-organisation of my subject once more; the intestine, I supposed, might come out quite on one side of it, as a half-tube, and the mesenchyme ring might be a half one also.

But things turned out as they were bound to do and not as I had expected; there was a typically *whole* gastrula on my dish the next morning, differing only by its small size from a normal one; and this *small but whole* gastrula was followed by a whole and typical small pluteus-larva (Fig. 5).

*a*_1 and *b*_1. Normal gastrula and normal pluteus.

*a*_2 and *b*_2. “Half”-gastrula and “half”-pluteus, that *ought* to result from one of the first two blastomeres, when isolated, according to the theory of “evolutio.”

*a*_3 and *b*_3. The small *but whole* gastrula and pluteus that actually *do* result.]

That was just the opposite of Roux’s result: one of the first two blastomeres had undergone a half-cleavage as in his case, but then it had become a whole organism by a simple process of rearrangement of its material, without anything that resembled regeneration, in the sense of a completion by budding from a wound.

If one blastomere of the two-cell stage was thus capable of performing the morphogenetical process in its totality, it became, of course, *impossible* to allow that nuclear division had separated any sort of “germ-plasm” into two different halves, and not even the protoplasm of the egg could be said to have been divided by the first cleavage furrow into unequal parts, as the postulate of the strict theory of so-called “evolutio” had been. This was a very important result, sufficient alone to overthrow at once the theory of ontogenetical “evolutio,” the “Mosaiktheorie” as it had been called--not by Roux himself, but according to his views--in its exclusiveness.

After first widening the circle of my observations by showing that one of the first four blastomeres is capable of performing a whole organogenesis, and that three of the first four blastomeres together result in an absolutely perfect organism, I went on to follow up separately one of the two fundamental problems which had been suggested by my first experiment: was there anything more to find out about the importance or unimportance of the single *nuclear* divisions in morphogenesis?[16]

[16] *Zeitschr. wiss. Zool.* 55, 1892.

By raising the temperature of the medium or by diluting the sea-water to a certain degree it proved at first to be possible to alter in a rather fundamental way the type of the cleavage-stages without any damage to the resulting organism. There may be no micromeres at the sixteen-cell stage, or they may appear as early as in the stage of eight cells; no matter, the larva is bound to be typical. So it certainly is not necessary for all the cleavages to occur just in their normal order.

But of greater importance for our purposes was what followed. I succeeded in pressing the eggs of Echinus between two glass plates, rather tightly, but without killing them; the eggs became deformed to comparatively flat plates of a large diameter. Now in these eggs all nuclear division occurred at right angles to the direction of pressure, that is to say, in the direction of the plates, as long as the pressure lasted; but the divisions began to occur at right angles to their former direction, as soon as the pressure ceased. By letting the pressure be at work for different times I therefore, of course, had it quite in my power to obtain cleavage types just as I wanted to get them. If, for instance, I kept the eggs under pressure until the eight-cell stage was complete, I got a plate of eight cells one beside the other, instead of two rings, of four cells each, one above the other, as in the normal case; but the next cell division occurred at right angles to the former ones, and a sixteen-cell stage, of two plates of eight cells each, one above the other, was the result. If the pressure continued until the sixteen-cell stage was reached, sixteen cells lay together in one plate, and two plates of sixteen cells each, one above the other, were the result of the next cleavage.

We are not, however, studying these things for cytological, but for morphogenetical purposes, and for these the cleavage phenomenon itself is less important than the organogenetic result of it: all our subjects resulted in *absolutely normal* organisms. Now, it is clear, that the spatial relations of the different nuclear divisions to each other are anything but normal, in the eggs subjected to the pressure experiments; that, so to say, every nucleus has got quite different neighbours if compared with the “normal” case. If that makes no difference, then there *cannot* exist any close relation between the single nuclear divisions and organogenesis at all, and the conclusion we have drawn more provisionally from the whole development of isolated blastomeres has been extended and proved in the most perfect manner. There ought to result a morphogenetic chaos according to the theory of real “evolutio” carried out by nuclear division, if the positions of the single nuclei were fundamentally changed with regard to one another (Fig. 6). But now there resulted not chaos, but the normal organisation: therefore it was disproved in the strictest way that nuclear divisions have any bearing on the origin of organisation; at least as far as the divisions during cleavage come into account.

*a*_1 and *b*_1. Two normal cleavage stages, consisting of eight and sixteen cells.

*a*_2 and *b*_2. Corresponding stages modified by exerting pressure until the eight-cell stage was finished. See text.]

On the egg of the frog (O. Hertwig), and on the egg of annelids (E. B. Wilson), my pressure experiments have been carried out with the same result.[17]

[17] In the pressure experiments I had altered the relative position of the nuclei *in origine*. In later years I succeeded in disturbing the arrangement of the fully formed cells of the eight-cell stage, and in getting normal larvæ in spite of that in many cases. But as this series of experiments is not free from certain complications--which in part will be understood later on (see page 73)--it must suffice here to have mentioned them. (For further information see my paper in *Archiv. f. Entwickelungsmechanik*, xiv., 1902, page 500.)

ON THE INTIMATE STRUCTURE OF THE PROTOPLASM OF THE GERM

Nuclear division, as we have seen, cannot be the basis of organogenesis, and all we know about the whole development of isolated blastomeres seems to show that there exists nothing responsible for differentiation in the protoplasm either.

But would that be possible? It cannot appear possible on a more profound consideration of the nature of morphogenesis, it seems to me: as the untypical agents of the medium cannot be responsible in any way for the origin of a form combination which is most typical and specific, there must be somewhere in the egg itself a certain factor which is responsible at least for the general orientation and symmetry of it. Considerations of this kind led me, as early as 1893,[18] to urge the hypothesis that there existed, that there *must* exist, a sort of intimate structure in the egg, including polarity and bilaterality as the chief features of its symmetry, a structure which belongs to every smallest element of the egg, and which might be imagined by analogy under the form of elementary magnets.[19] This hypothetic structure could have its seat in the protoplasm only. In the egg of echinoderms it would be capable of such a quick rearrangement after being disturbed, that it could not be observed but only inferred logically; there might, however, be cases in which its real discovery would be possible. Indeed Roux’s frog-experiment seems to be a case where it is found to be at work: at least it seems very probable to assume that Roux obtained half of a frog’s embryo because the protoplasm of the isolated blastomere had preserved the “halfness” of its intimate structure, and had not been able to form a small whole out of it.

[18] *Mitteil. Neapel. 11, 1893.*

[19] But the elementary magnets would have to be bilateral!

Of course it was my principal object to verify this hypothesis, and such verification became possible in a set of experiments which my friend T. H. Morgan and myself carried out together,[20] in 1895, on the eggs of ctenophores, a sort of pelagic animals, somewhat resembling the jelly-fish, but of a rather different inner organisation. The zoologist Chun had found even before Roux’s analytical studies, that isolated blastomeres of the ctenophore egg behave like parts of the whole and result in a half-organisation like the frog’s germ does. Chun had not laid much stress on his discovery, which now, of course, from the new points of view, became a very important one. We first repeated Chun’s experiment and obtained his results, with the sole exception that there was a tendency of the endoderm of the half-larva of Beroë to become more than “half.” But that was not what we chiefly wanted to study. We succeeded in cutting away a certain mass of the protoplasm of the ctenophore egg just before it began to cleave, without damaging its nuclear material in any way: in all cases, where the cut was performed at the side, there resulted a certain type of larvae from our experiments which showed exactly the same sort of defects as were present in larvae developed from one of the first two blastomeres alone.

[20] *Arch. Entw. Mech.* 2, 1895.

The hypothesis of the morphogenetic importance of *protoplasm* had thus been proved. In our experiments there was all of the nuclear material, but there were defects on one side of the protoplasm of the egg; and the defects in the adult were found to correspond to these defects in the protoplasm.

And now O. Schultze and Morgan succeeded in performing some experiments which directly proved the hypothesis of the part played by protoplasm in the subject employed by Roux, *viz.*, the frog’s egg. The first of these investigators managed to rear two whole frog embryos of small size, if he slightly pressed the two-cell stage of that form between two plates of glass and turned it over; and Morgan,[21] after having killed one of the first two blastomeres, as was done in the original experiment of Roux, was able to bring the surviving one to a half or to a whole development according as it was undisturbed or turned. There cannot be any doubt that in both of these cases, it is the possibility of a rearrangement of protoplasm, offered by the turning over, which allows the isolated blastomere to develop as a whole. The regulation of the frog’s egg, with regard to its becoming whole, may be called facultative, whilst the same regulation of the egg of Echinus is obligatory. It is not without interest to note that the first two blastomeres of the common newt, *i.e.* of a form which belongs to the other class of Amphibia, after a separation of *any* kind, *always* develop as wholes, their faculty of regulation being obligatory, like that of Echinus.

[21] *Anat. Anz.* 10, 1895.

Whole or partial development may thus be dependent on the power of regulation contained in the intimate polar-bilateral structure of the protoplasm. Where this is so, the regulation and the differences in development are both connected with the chief relations of symmetry. The development becomes a half or a quarter of the normal because there is only one-half or one-quarter of a certain structure present, one-half or one-quarter with regard to the very wholeness of this structure; the development is whole, in spite of disturbances, if the intimate structure became whole first. We may describe the “wholeness,” “halfness,” or “quarterness” of our hypothetic structure in a mathematical way, by using three axes, at right angles to one another, as the base of orientation. To each of these, *x*, *y*, and *z*, a certain specific state with regard to the symmetrical relations corresponds; thence it follows that, if there are wanting all those parts of the intimate structure which are determined, say, by a negative value of *y*, by minus *y*, then there is wanting half of the intimate structure; and this halfness of the intimate structure is followed by the halfness of organogenesis, the dependence of the latter on the intimate structure being established. But if regulation has restored, on a smaller scale, the whole of the arrangement according to all values of *x*, *y* and *z*, development also can take place completely (Fig. 7).

The large circle represents the original structure of the egg. In all cases where cleavage-cells of the two-cell stage are isolated this original structure is only present as “half” in the beginning, say only on the right (+*y*) side. Development then becomes “half,” if the intimate structure remains half; but it becomes “whole” (on a smaller scale) if a new whole-structure (small circle!) is formed by regulatory processes.]

I am quite aware that such a discussion is rather empty and purely formal, nevertheless it is by no means without value, for it shows most clearly the differences between what we have called the intimate structure of germs, responsible only for the general symmetry of themselves and of their isolated parts, and another sort of possible structure of the egg-protoplasm which we now shall have to consider, and which, at the first glance, seems to form a serious difficulty to our statements, as far at least as they claim to be of general importance. The study of this other sort of germinal structure at the same time will lead us a step farther in our historical sketch of the first years of “Entwickelungsmechanik” and will bring this sketch to its end.

ON SOME SPECIFICITIES OF ORGANISATION IN CERTAIN GERMS

It was known already about 1890, from the careful study of what has been called “cell-lineage,” that in the eggs of several families of the animal kingdom the origin of certain organs may be traced back to individual cells of cleavage, having a typical histological character of their own. In America especially such researches have been carried out with the utmost minuteness, E. B. Wilson’s study of the cell-lineage of the Annelid *Nereis* being the first of them. If it were true that nuclear division is of no determining influence upon the ontogenetic fate of the blastomeres, only peculiarities of the different parts of the protoplasm could account for such relations of special cleavage cells to special organs. I advocated this view as early as in 1894, and it was proved two years later by Crampton, a pupil of Wilson’s, in some very fine experiments performed on the germ of a certain mollusc.[22] The egg of this form contains a special sort of protoplasm near its vegetative pole, and this part of it is separated at each of the first two segmentations by a sort of pseudo-cleavage, leading to stages of three and five separated masses instead of two and four, the supernumerary mass being the so-called “yolk-sac” and possessing no nuclear elements (Fig. 8). Crampton removed this yolk-sac at the two-cell stage, and he found that the cleavage of the germs thus operated upon was normal except with regard to the size and histological appearance of one cell, and that the larvae originating from these germs were complete in every respect except in their mesenchyme, which was wanting. A special part of the protoplasm of the egg had thus been brought into relation with quite a special part of organisation, *and that special part of the protoplasm contained no nucleus*.

[22] *Arch. Entw. Mech.* 3, 1896.

*a.* The egg, consisting of three different kinds of protoplasmatic material.

*b.* First cleavage-stage. There are two cells and one “pseudo-cell,” the yolk-sac, which contains no nucleus. This was removed in Crampton’s experiment.]

GENERAL RESULTS OF THE FIRST PERIOD OF “ENTWICKELUNGSMECHANIK”

This experiment of Crampton’s, afterwards confirmed by Wilson himself, may be said to have closed the first period of the new science of physiology of form, a period devoted almost exclusively to the problem whether the theory of nuclear division or, in a wider sense, whether the theory of a strict “evolutio” as the basis of organogenesis was true or not.

It was shown, as we have seen, that the theory of the “qualitatively unequal nuclear division” (“qualitativ-ungleiche Kernteilung” in German) certainly was not true, and that there also was no strict “evolutio” in protoplasm. Hence Weismann’s theory was clearly disproved. There certainly is a good deal of real “epigenesis” in ontogeny, a good deal of “production of manifoldness,” not only with regard to visibility but in a more profound meaning. But some sort of pre-formation had also been proved to exist, and this pre-formation, or, if you like, this restricted evolution, was found to be of two different kinds. First an intimate organisation of the protoplasm, spoken of as its polarity and bilaterality, was discovered, and this had to be postulated for every kind of germs, even when it was overshadowed by immediate obligatory regulation after disturbances. Besides that there were cases in which a real specificity of special parts of the germ existed, a relation of these special parts to special organs: but this sort of specification also was shown to belong to the protoplasm.

It follows from all we have mentioned about the organisation of protoplasm and its bearing on morphogenesis, that the eggs of different animals may behave rather differently, in this respect, and that the eggs indeed may be classified according to the degree of their organisation. Though we must leave a detailed discussion of these topics to morphology proper, we yet shall try shortly to summarise what has been ascertained about them in the different classes of the animal kingdom. A full regulation of the *intimate* structure of isolated blastomeres to a new whole, has been proved to exist in the highest degree in the eggs of all echinoderms, medusae, nemertines, Amphioxus, fishes, and in one class of the Amphibia (the *Urodela*); it is facultative only among the other class of Amphibia, the *Anura*, and seems to be only partly developed or to be wanting altogether among ctenophora, ascidia, annelids, and mollusca. Peculiarities in the organisation of *specific parts* of protoplasm have been proved to occur in more cases than at first had been assumed; they exist even in the echinoderm egg, as experiments of the last few years have shown; even here a sort of specification exists at the vegetative pole of the egg, though it is liable to a certain kind of regulation; the same is true in medusae, nemertines, etc.; but among molluscs, ascidians, and annelids no regulation about the specific organisation of the germ in cleavage has been found in any case.

The differences in the degree of regulability of the intimate germinal structure may easily be reduced to simple differences in the physical consistency of their protoplasm.[23] But all differences in specific organisation must remain as they are for the present; it will be one of the aims of the future theory of development to trace these differences also to a common source.

[23] It deserves notice in this connection, that in some cases the protoplasm of parts of a germ has been found to be more regulable in the earliest stages, when it is very fluid, than later, when it is more stiff.

That such an endeavour will probably be not without success, is clear, I should think, from the mere fact that differences with regard to germinal specific pre-formation do not agree in any way with the systematic position of the animals exhibiting them; for, strange as it would be if there were two utterly different kinds of morphogenesis, it would be still more strange if there were differences in morphogenesis which were totally unconnected with systematic relationship: the ctenophores behaving differently from the medusae, and Amphioxus differently from ascidians.

SOME NEW RESULTS CONCERNING RESTITUTIONS

We now might close this chapter, which has chiefly dealt with the disproof of a certain sort of ontogenetic theories, and therefore has been almost negative in its character, did it not seem desirable to add at least a few words about the later discoveries relating to morphogenetic restorations of the adult. We have learnt that Weismann created his concept of “reserve plasma” to account for what little he knew about “restitutions”: that is, about the restoration of lost parts: he only knew regeneration proper in animals and the formation of adventitious buds in plants. It is common to both of these phenomena that they take their origin from typically localised points of the body in every case; each time they occur a certain well-defined part of the body is charged with the restoration of the lost parts. To explain such cases Weismann’s hypothesis was quite adequate, at least in a logical sense. But at present, as we shall discuss more fully in another chapter, we know of some very widespread forms of restitution, in which what is to be done for a replacement of the lost is not entrusted to *one* typical part of the body in every case, but in which the whole of the morphogenetic action to be performed is transferred in its *single* parts to the *single* parts of the body which is accomplishing restoration: each of its parts has to take an individual share in the process of restoration, effecting what is properly called a certain kind of “re-differentiation” (“Umdifferenzierung”), and this share varies according to the relative position of the part in each case. Later on these statements will appear in more correct form than at present, and then it will become clear that we are fully entitled to emphasise at the end of our criticism of Weismann’s theory, that his hypothesis relating to restorations can be no more true than his theory of development proper was found to be.

And now we shall pass on to our positive work.

We shall try to sketch the outlines of what might properly be called an *analytical theory of morphogenesis*; that is, to explain the sum of our knowledge about organic form-production, gained by experiment and by logical analysis, in the form of a real system, in which each part will be, or at least will try to be, in its proper place and in relation with every other part. Our analytical work will give us ample opportunity of mentioning many important topics of so-called general physiology also, irrespective of morphogenesis as such. But morphogenesis is always to be the centre and starting-point of our analysis. As I myself approach the subject as a zoologist, animal morphogenesis, as before, will be the principal subject of what is to follow.

2. ANALYTICAL THEORY OF MORPHOGENESIS[24]

[24] Compare my *Analytische Theorie der organischen Entwickelung*, Leipzig, 1894, and my reviews in *Ergebnisse der Anatomie und Entwickelungsgeschichte*, vols. viii. xi. xiv., 1899-1905. A shorter review is given in *Ergebnisse der Physiologie*, vol. v., 1906. The full literature will be found in these reviews.

α. THE DISTRIBUTION OF MORPHOGENETIC POTENCIES

*Prospective Value and Prospective Potency*

Wilhelm Roux did not fail to see that the questions of the locality and the time of all morphogenetic differentiations had to be solved first, before any problem of causality proper could be attacked. From this point of view he carried out his fundamental experiments.

It is only in terminology that we differ from his views, if we prefer to call our introductory chapter an analysis of the distribution of morphogenetic potencies. The result will be of course rather different from what Roux expected it would be.

Let us begin by laying down two fundamental concepts. Suppose we have here a definite embryo in a definite state of development, say a blastula, or a gastrula, or some sort of larva, then we are entitled to study any special element of any special elementary organ of this germ with respect to what is actually to develop out of this very element in the future actual course of this development, whether it be undisturbed or disturbed in any way; it is, so to say, the actual, *the real fate* of our element, that we take in account. I have proposed to call this real fate of each embryonic part in this very definite line of morphogenesis its *prospective value* (“prospective Bedeutung” in German). The fundamental question of the first chapter of our analytical theory of development may now be stated as follows: Is the prospective value of each part of any state of the morphogenetic line constant, *i.e.* is it unchangeable, can it be nothing but one; or is it variable, may it change according to different circumstances?

We first introduce a second concept: the term *prospective potency* (“prospective Potenz” in German) of each embryonic element. The term “prospective morphogenetic potency” is to signify the *possible fate* of each of those elements. With the aid of our two artificial concepts we are now able to formulate our introductory question thus: Is the prospective potency of each embryonic part fully given by its prospective value in a certain definite case; is it, so to say, identical with it, or does the prospective potency contain more than the prospective value of an element in a certain case reveals?

We know already from our historical sketch that the latter is true: that the actual fate of a part need not be identical with its possible fate, at least in many cases; that the potency of the first four blastomeres of the egg of the sea-urchin, for instance, has a far wider range than is shown by what each of them actually performs in even this ontogeny. There are more morphogenetic possibilities contained in each embryonic part than are actually realised in a special morphogenetic case.

As the most important special morphogenetic case is, of course, the so-called “normal” one, we can also express our formula in terms of special reference to it: there are more morphogenetic possibilities in each part than the observation of the normal development can reveal. Thus we have at once justified the application of analytical experiment to morphogenesis, and have stated its most important results.

As the introductory experiments about “Entwickelungsmechanik” have shown already that the prospective potency of embryonic parts, at least in certain cases, *can* exceed their prospective value--that, at least in certain cases, it can be different from it--the concept of prospective potency at the very beginning of our studies puts itself in the centre of analytical interest, leaving to the concept of prospective value the second place only. For that each embryonic part actually has a certain prospective value, a specified actual fate in every single case of ontogeny, is clear from itself and does not affirm more than the reality of morphogenetic cases in general; but that the prospective value of the elements may change, that there is a morphogenetic power in them, which contains more than actuality; in other words, that the term “prospective potency” has not only a logical but a factual interest: all these points amount to a statement not only of the most fundamental introductory results but also of the actual *problems* of the physiology of form.

If at each point of the germ something else *can* be formed than actually is formed, why then does there happen in each case just what happens and nothing else? In these words indeed we may state the chief problem of our science, at least after the fundamental relation of the superiority of prospective potency to prospective value has been generally shown.

We consequently may shortly formulate our first problem as the question of the distribution of the prospective morphogenetic potencies in the germ. Now this general question involves a number of particular ones. Up to what stage, if at all, is there an absolutely equal distribution of the potencies over all the elements of the germ? When such an equal distribution has ceased to exist at a certain stage, what are then the relations between the parts of different potency? How, on the other hand, does a newly arisen, more specialised sort of potency behave with regard to the original general potency, and what about the distribution of the more restricted potency?

I know very well that all such questions will seem to you a little formal, and, so to say, academical at the outset. We shall not fail to attach to them very concrete meanings.

*The Potencies of the Blastomeres*

At first we turn back to our experiments on the egg of the sea-urchin as a type of the germ in the very earliest stages. We know already that each of the first two, or each of the first four, or three of the first four blastomeres together may produce a whole organism. We may add that the swimming blastula, consisting of about one thousand cells, when cut in two quite at random, in a plane coincident with, or at least passing near, its polar axis, may form two fully developed organisms out of its halves.[25] We may formulate this result in the words: the prospective potency of the single cells of a blastula of Echinus is the same for all of them; their prospective value is as far as possible from being constant.

[25] If the plane of section passes near the equator of the germ, two whole larvae may be formed also, but in the majority of cases the “animal” half does not go beyond the blastula. The specific features of the organisation of the protoplasm come into account here. See also page 65, note 1.

But we may say even a little more: what actually will happen in each of the blastula cells in any special case of development experimentally determined depends on the position of that cell in the whole, if the “whole” is put into relation with any fixed system of co-ordinates; or more shortly, “the prospective value of any blastula cell is a function of its position in the whole.”

I know from former experience that this statement wants a few words of explanation. The word “function” is employed here in the most general, mathematical sense, simply to express that the prospective value, the actual fate of a cell, will change, whenever its position in the whole is different.[26] The “whole” may be related to any three axes drawn through the normal undisturbed egg, on the hypothesis that there exists a primary polarity and bilaterality of the germ; the axes which determine this sort of symmetry may, of course, conveniently be taken as co-ordinates; but that is not necessary.

[26] A change of the position of the cell is of course effected by each variation of the direction of the cut, which is purely a matter of chance.

*The Potencies of Elementary Organs in General*

Before dealing with other very young germs, I think it advisable to describe first an experiment which is carried out at a later stage of our well-known form. This experiment will easily lead to a few new concepts, which we shall want later on, and will serve, on the other hand, as a basis of explanation for some results, obtained from the youngest germs of some other animal species, which otherwise would seem to be rather irreconcilable with what our Echinus teaches us.

You know, from the second lecture, what a gastrula of our sea-urchin is. If you bisect this gastrula, when it is completely formed, or still better, if you bisect the gastrula of the starfish, either along the axis or at right angles to it, you get complete little organisms developed from the parts: the ectoderm is formed in the typical manner in the parts, and so is the endoderm; everything is proportionate and only smaller than in the normal case. So we have at once the important results, that, as in the blastula, so in the ectoderm and in the endoderm of our Echinus or of the starfish, the prospective potencies are the same for every single element: both in the ectoderm and in the endoderm the prospective value of each cell is a “function of its position” (Fig. 9).

*a*^1. Normal gastrula; may be bisected along the main axis or at right angles to it (see dotted lines).

*a*^2. Normal larva, “*Bipinnaria*.”

*b*^1. Small but whole gastrula that results by a process of regulation from the parts of a bisected gastrula.

*b*^2. Small *but whole* “*Bipinnaria*,” developed out of *b*^1.]

But a further experiment has been made on our gastrula. If at the moment when the material of the future intestine is most distinctly marked in the blastoderm, but not yet grown into a tube, if at this moment the upper half of the larva is separated from the lower by an equatorial section, you will get a complete larva only from that part which bears the “Anlage” of the endoderm, while the other half will proceed in morphogenesis very well but will form only ectodermal organs. By another sort of experiment, which we cannot fully explain here, it has been shown that the endoderm if isolated is also only able to form such organs as are normally derived from it.

And so we may summarise both our last results by saying: though ectoderm and endoderm have their potencies equally distributed amongst their respective cells, they possess different potencies compared one with the other. And the same relation is found to hold for all cases of what we call elementary organs: they are “equipotential,” as we may say, in themselves, but of different potencies compared with each other.

*Explicit and Implicit Potencies: Primary and Secondary Potencies*

We shall first give to our concept of “prospective potency” a few words of further analytical explanation with the help of our newly obtained knowledge.

It is clear from what we have stated that the prospective potencies of the ectoderm and of the endoderm, and we may add, of every elementary organ in relation to every other, differ between themselves and also in comparison with the blastoderm, from which they have originated. But the diversity of the endoderm with respect to the ectoderm is not of the same kind as its diversity in respect to the blastoderm. The potency of the endoderm and that of the ectoderm are both specialised in their typical manner, but compared with the potency of the blastoderm they may be said not only to be specialised but also to be *restricted*: the potency of the blastoderm embraces the whole, that of the so-called germ-layer embraces only part of the whole; and this species of restriction becomes clearer and clearer the further ontogeny advances: at the end of it in the “ultimate elementary organs” there is no prospective potency whatever.

A few new terms will serve to state a little more accurately what happens. Of course, with regard to all morphogenesis which goes on *immediately* from the blastoderm, the potency of the blastoderm is restricted as much as are the potencies of the germ layers. We shall call this sort of immediate potency *explicit*, and then we see at once that, with regard to their explicit potencies, there are only differences among the prospective potencies of the elementary organs; but with respect to the *implicit* potency of any of these organs, that is with respect to their potency as embracing the faculties of all their derivations, there are also not only differences but true morphogenetic restrictions lying at the very foundations of all embryology.

But now those of you who are familiar with morphogenetic facts will object to me, that what we have stated about all sorts of restrictions in ontogeny is not true, and you will censure me for having overlooked regeneration, adventitious budding, and so on. To some extent the criticism would be right, but I am not going to recant; I shall only introduce another new concept. We are dealing only with *primary* potencies in our present considerations, *i.e.* with potencies which lie at the root of true embryology, not with those serving to regulate disturbances of the organisation. It is true, we have in some way disturbed the development of our sea-urchin’s egg in order to study it; more than that, it would have been impossible to study it at all without some sort of disturbance, without some sort of operation. But, nevertheless, no potencies of what may properly be called the *secondary* or restitutive type have been aroused by our operations; nothing happened except on the usual lines of organogenesis. It is true, some sort of regulation occurred, but that is included among the factors of ontogeny proper.

We shall afterwards study more fully and from a more general point of view this very important feature of “primary regulation” in its contrast to “secondary regulation” phenomena. At present it must be enough to say that in speaking of the restriction of the implicit potencies in form-building we refer only to potencies of the primary type, which contain within themselves some properties of a (primary) regulative character.

*The Morphogenetic Function of Maturation in the Light of Recent Discoveries*

Turning again to more concrete matters, we shall first try, with the knowledge acquired of the potencies of the blastoderm and the so-called germ layers of Echinus, to understand certain rather complicated results which the experimental morphogenetic study of other animal forms has taught us. We know from our historical sketch that there are some very important aberrations from the type, to which the Echinus germ belongs,[27] *i.e.* the type with an equal distribution of the potencies over all the blastomeres. We know not only that in cases where a regulation of the intimate structure of the protoplasm fails to occur a partial development of isolated cells will take place, but that there may even be a typical disposition of typical cells for the formation of typical organs only, without any regulability.

[27] The reader will remember (see page 65, note 1), that even the germ of Echinus is not quite equipotential along its main axis, but it is equipotential in the strictest sense around this axis. The germs of certain medusae seem to be equipotential in every respect, even in their cleavage stages.

Let us first consider the last case, of which the egg of mollusca is a good type: here there is no equal distribution of potencies whatever, the cleavage-cells of this germ are a sort of real “mosaic” with regard to their morphogenetic potentialities. Is this difference between the germ of the echinoderms and the molluscs to remain where it is, and not to be elucidated any further? Then there would be rather important differences among the germs of different animals, at least with regard to the degree of the specification of their cleavage cells, or if we ascribe differences among the blastomeres to the organisation of the fertilised egg ready for cleavage, there would be differences in the morphogenetic organisation of the egg-protoplasm: some eggs would be more typically specialised at the very beginning of morphogenesis than others.

In the first years of the study of “Entwickelungsmechanik” I pointed out that it must never be forgotten that the egg itself is the result of organogenesis. If, therefore, there are real mosaic-like specifications in some eggs at the beginning of cleavage, or during it, there may perhaps have been an *earlier* stage in the individual history of the egg which did not show such specifications of the morphogenetic structure. Two American authors share the merit of having proved this hypothesis. Conklin showed, several years ago, that certain intracellular migrations and rearrangements of material do happen in the first stages of ovogenesis in certain cases, but it is to E. B. Wilson[28] that science owes a proper and definitive elucidation of the whole subject. Wilson’s researches, pursued not only by descriptive methods,[29] but also by means of analytical experiment, led him to the highly important discovery that the eggs of several forms (nemertines, molluscs), which after maturation show the mosaic type of specification in their protoplasm to a more or less high degree, fail to show any kind of specification in the distribution of their potencies before maturation has occurred. In the mollusc egg a certain degree of specification is shown already before maturation, but nothing to be compared with what happens afterwards; in the egg of nemertines there is no specification at all in the unripe egg.

[28] *Journ. Exp. Zool.* 1, 1904.

[29] Great caution must be taken in attributing any specific morphogenetic part to differently coloured or constructed materials, which may be observed in the egg-protoplasm in certain cases. They may play such a part, but in other cases they certainly do not (see Lyon, *Arch. Entw. Mech.* 23, 1907). The final decision always depends on experiment.

Maturation thus becomes a part of ontogeny itself; it is not with fertilisation that morphogenesis begins, there is a sort of ontogeny anterior to fertilisation.

These words constitute a summary of Wilson’s researches. Taken together with the general results obtained about the potencies of the blastula and the gastrula of Echinus, they reduce what appeared to be differences of degree or even of kind in the specification of the egg-protoplasm *to mere differences in the time of the beginning of real morphogenesis*. What occurs in some eggs, as in those of Echinus, at the time of the definite formation of the germ layers, leading to a specification and restriction of their prospective potencies, may happen very much earlier in other eggs. But there exists in *every* sort of egg an *earliest* stage, in which all parts of its protoplasm are equal as to their prospectivity, and in which there are no potential diversities or restrictions of any kind.

So much for differences in the *real material* organisation of the germ and their bearing on inequipotentialities of the cleavage cells.

*The Intimate Structure of Protoplasm: Further Remarks*

Where a typical half- or quarter-development from isolated blastomeres happens to occur, we know already that the impossibility of a regulation of the *intimate polar-bilateral* structure may account for it. As this impossibility of regulation probably rests on rather simple physical conditions[30] it may properly be stated that equal distribution of potencies is not wanting but is only overshadowed here. In this respect there exists a logical difference of fundamental importance between those cases of so-called “partial” or better, “fragmental” development of isolated blastomeres in which a certain embryonic organ is wanting on account of its specific morphogenetic material being absent, and those cases in which the “fragmental” embryo lacks complete “halves” or “quarters” with regard to general symmetry on account of the symmetry of its intimate structure being irregularly disturbed. This logical difference has not always received the attention which it undoubtedly deserves. Our hypothetical intimate structure in itself is, of course, also a result of factors concerned in ovogenesis. Only in one case do we actually know anything about its origin: Roux has shown that in the frog it is the accidental path of the fertilising spermatozoon in the egg which, together with the polar axis, normally determines the plane of bilateral symmetry; but this symmetry may be overcome and replaced by another, if gravity is forced to act in an abnormal manner upon the protoplasm; the latter showing parts of different specific gravity in the eggs of all Amphibia.

[30] It seems that these physical conditions also--besides the real specifications in the organisation of the egg--may be different before and after maturation or (in other cases) fertilisation. (See Driesch, *Archiv f. Entwickelungsmechanik*, 7, p. 98; and Brachet, *ibid.* 22, p. 325.)

*The Neutrality of the Concept of “Potency”*

Now we may close our rather long chapter on the distribution of potencies in the germ; it has been made long, because it will prove to be very important for further analytical discussion; and its importance, in great measure, is due to its freedom from prepossessions. Indeed, the concept of prospective potency does not prejudice anything; we have said, it is true, that limitations of potencies may be due to the presence of specific parts of organisation in some cases; that, at least, they may be connected therewith; but we have not determined at all what a prospective potency really is, what the term really is to signify. It may seem that such a state of things gives an air of emptiness to our discussions, that it leaves uncertain what is the most important. But, I think, our way of argument, which tries to reach the problems of greatest importance by degrees, though it may be slow, could hardly be called wrong and misleading.

β. THE “MEANS” OF MORPHOGENESIS

We now proceed to an analysis of what may properly be called the *means* of morphogenesis, the word “means” being preferable to the more usual one “conditions” in this connection, as the latter would not cover the whole field. It is in quite an unpretentious and merely descriptive sense that the expression “means” should be understood at present; what is usually called “conditions” is part of the morphogenetic means in our sense.

β′. *The Internal Elementary Means of Morphogenesis*

We know that all morphogenesis, typical or atypical, primary or secondary, goes on by one morphogenetic elementary process following the other. Now the very foundation of these elementary processes themselves lies in the elementary functions of the organism as far as they result in the formation of stable visible products. Therefore the elementary functions of the organism may properly be called the internal “means” of morphogenesis.

Secretion and migration are among such functions; the former happening by the aid of chemical change or by physical separation, the latter by the aid of changes in surface tension. But hardly anything more concrete has been made out about these or similar points at present.

We therefore make no claim to offer a complete system of the internal elementary means of morphogenesis. We shall only select from the whole a few topics of remarkable morphogenetic interest, and say a few words about each.

But, first of all, let us observe that the elementary means of morphogenesis are far from being morphogenesis themselves. The word “means” itself implies as much. It would be possible to understand each of these single acts in morphogenesis as well as anything, and yet to be as far from understanding the whole as ever. All means of morphogenesis are only to be considered as the most general frame of events within which morphogenesis occurs.

*Some Remarks on the Importance of Surface Tension in Morphogenesis.*--There are a few purely physical phenomena which have a special importance in organic morphology, all of them connected with capillarity or surface tension. Soap-lather is a very familiar thing to all of you: you know that the soap-solution is arranged here in very thin planes separated by spaces containing air: it was first proved by Berthold[31] that the arrangement of cells in organic tissues follows the same type as does the arrangement of the single bubbles of a soap-lather, and Bütschli[32] added to this the discovery that the minute structure of the protoplasm itself is that of a foam also. Of course it is not one fluid and one gas which make up the constituents of the structure in the organisms, as is the case in the well-known inorganic foams, but two fluids, which do not mix with one another. One general law holds for all arrangements of this kind: the so-called law of least surfaces, expressed by the words that the sum of all surfaces existing is a minimum; and it again is a consequence of this law, if discussed mathematically, that four lines will always meet in one point and three planes in one line. This feature, together with a certain law about the relation of the angles meeting in one line to the size of the bubbles, is realised most clearly in many structures of organic tissues, and makes it highly probable, at least in some cases, that capillarity is at work here. In other cases, as for instance in many plants, a kind of outside pressure, the so-called tissue tension, may account for the arrangement in surfaces *minimae areae*. Cleavage stages are perhaps the very best type in which our physical law is expressed: and here it may be said to have quite a simple application whenever all of the blastomeres are of the same physical kind, whilst some complications appear in germs with a specialised organisation and, therefore, with differences in the protoplasm of their single blastomeres. In such instances we may say that the physical law holds as far as the conditions of the system permit, these conditions ordinarily consisting in a sort of non-homogeneity of the surfaces.

[31] *Studien über Protoplasmamechanik*, Leipzig, 1886.

[32] *Unters. üb. mikroskopische Schäume und das Protoplasma*, Leipzig, 1892.

It seems, from the researches of Dreyer,[33] that the formation of organic skeletons may also be governed by the physically conditioned arrangement of protoplasmatic or cellular elements, and some phenomena of migration and rearrangement among cleavage cells, as described by Roux, probably also belong here.

[33] *Jena. Zeitschr.* 26, 1892.