It may be pointed out that while more recent work has substantiated, on the whole, the latter conclusions[125] of Pflüger, just stated, still the results of studies of regenerative phenomena of organisms show that the conclusions are not necessarily the only ones deducible from the experiments; for, although it may be true that any possible primary meridian of the egg may become the median plane of the body of the embryo, it does not follow that there is no one organized plane always present in the normal egg, i.e. the egg may not be entirely isotropic. That this may be the case is shown in the regeneration of pieces of adult animals in which a piece cut to one side of the old median plane may develop a new plane of symmetry of its own. This possibility must be also admitted for the egg. If we substitute the term “totipotence,” meaning that any meridian of the egg has the possibility of becoming the median plane of the embryo, in place of Pflüger’s term “isotropy,” we remove this element of possible error from his statement.
Roux and Born have shown that the only action that gravity has on the frog’s egg is to bring about a rearrangement of the contents of the egg, a phenomenon that Pflüger had not observed. The lighter part flows to the highest region of the egg, and the heaviest to the bottom of the egg, hence the change in the position of the cleavage planes observed by Pflüger that begin in the upper, more protoplasmic part of the egg.
Another series of experiments, that we also owe, in the first place, to Pflüger (’84), consist in compressing the egg before and during its cleavage. The position of several of the cleavage planes may be altered, yet a normal embryo develops from the egg. The same experiment has been repeated by Hertwig (’93), and by Born (’93), on the frog’s egg, and by Driesch (’92), Ziegler (’94), myself (’93), and others, on the egg of the sea-urchin, with substantially the same results. The value of the experiment lies not so much in showing that the coincidence between the first cleavage planes and the orienting planes of the body may be lost, as in showing that under these circumstances the nuclei have a different distribution in the protoplasm from that which they hold in the normal egg. Any theory of development that depends on the qualitative distribution of nuclear products during the cleavage period meets with great difficulties in the light of these results, and in order to overcome them will be obliged to add qualifications of such a kind as materially to alter its simplicity. Roux’s theory, for instance, comes into this category. Roux (’83) suggested that since the complicated karyokinetic division of the nucleus is carried out in such a way as to insure a precise division of the chromatin, and since the qualities of the male are transmitted to the egg through the chromatin of the spermatozoon, it is probable that the division of the chromatin is a qualitative process, by means of which the elements are distributed to different parts of the egg. According to Roux, the first division of the frog’s egg divides the material of the right half of the embryo from that of the left; the second division separates the material of the anterior half from that of the posterior half. Roux limited, to a certain extent, his hypothesis to these two divisions of the frog’s egg, and stated further that it is not improbable that during the later stages of development there may take place an interaction of the parts on each other, and this interaction would be another factor in the development. Weismann has adopted Roux’s hypothesis, and has extended it to all organisms, and to most of the divisions of the developing egg, at least to all those divisions in which the qualities of the layers, tissues, organs, etc., are separated. On this slight basis he has constructed his theory of development and of regeneration. It is important, therefore, to examine critically the evidence furnished by experimental embryology for or against this hypothesis of a qualitative division of the egg during the cleavage period.
The development of a half embryo from one of the first two blastomeres of the frog’s egg, in Roux’s experiment, seemed to support Roux’s hypothesis, but it was not long before it was seen that the presence of the other blastomere vitiated the evidence to such an extent as to render it worthless, so far as this hypothesis is concerned. Then followed the experiments with the isolated blastomeres of the sea-urchin, amphioxus, jelly-fish, teleost, ascidian, triton, etc., in which each blastomere, when completely separated, gives rise to a whole embryo. From these experiments Driesch and Hertwig drew the opposite conclusion, namely, that during the cleavage there is a quantitative division of the egg into blastomeres that are equivalent, or at least totipotent. Roux attempted to meet the results of these experiments in two ways. He pointed out that in several of these cases the isolated blastomere divides as a half or as a fourth of the egg, and that in the sea-urchin this leads to the formation of an open half-blastula. In the second place, Roux brought more to the front his subsidiary hypothesis of the reserve germ plasm. He supposed that along with the early qualitative division of the nucleus, by means of which each part receives its particular chromatic substance, there is also a quantitative division of a sort of reserve germ plasm contained in the nucleus. Each cell may receive also a part of this material, and hence each cell may contain the potentialities of the whole egg. This reserve plasm may be awakened by any change that alters the normal development, as, for instance, when the blastomeres are separated. It may take some time for this reserve stuff to wake up, as shown by the half-development of the sea-urchin’s egg that goes on for some time after the separation of the blastomeres. This hypothesis cannot be objected to on purely formal grounds, but we are not so much concerned with a purely logical hypothesis as with a verifiable one.
It has been pointed out that the experiment of compressing the egg in different planes that leads to a new distribution of the nuclei is a formidable obstacle to Roux’s hypothesis. If the nuclear divisions in the compressed egg are of the same sort as in the normal egg, we should expect to find as a result either a monstrous form with all its parts misplaced, or, if the parts are mutually dependent, nothing at all. Roux has attempted to meet this case by supposing that the nucleus itself responds to the change in the protoplasm and alters its divisions in such a way as to send to each part of the compressed egg the right sort of material for that part. This means that the nucleus can so entirely change the sequence of its divisions that instead, for instance, of sending to each pole of the first spindle the material of the right and left sides of the body, as it does normally, it may divide under compression in such a way that the material for the anterior half of the embryo is separated from that of the posterior half. That a change involving such a vast number of qualities could take place, as a result of the slight compression on the egg that brings about a change in the position of the spindle, seems highly improbable. It is, of course, not a disproof of the hypothesis to show that it involves very great complications, for the very assumption itself of a qualitative division of the nucleus, in the Roux-Weismann sense, involves us in great complications.
A more damaging criticism of the hypothesis of a qualitative division of the nucleus is found in an appeal to direct observation, which shows that the chromatin divides always into exactly equal parts. In many cases we know, from the subsequent fate of the cells, that two cells arising from the same cell play very different rôles in the subsequent development, yet the chromatin of the nucleus is always divided equally.
The development of the isolated blastomeres of the ctenophore egg may seem at first sight to give support to Roux’s hypothesis, for in this case the first two cells are completely separated, and yet give rise to half-structures. Crampton’s experiments on the eggs of ilyanassa may also appear to be evidence in favor of this view. In fact, however, they give no more support to the idea of a qualitative division of the nucleus than they do to that of a qualitative division in the protoplasm, and there are some further experiments on the ctenophore egg which indicate that it is the latter rather than the former sort of division that takes place. As stated in the preceding chapter, Driesch and Morgan found that, if a part of the protoplasm of the unsegmented egg of the ctenophore is removed, an incomplete embryo develops, although the whole of the segmentation nucleus is present. Ziegler’s results show that, even after the removal of that part of the egg from which the micromeres develop, the segmentation may still be like that of the whole egg, and this shows that the egg has great powers of recuperation (at least in a symmetrical plane), so far as its protoplasm is concerned. If, however, it is true that when a part is cut off unsymmetrically the protoplasm cannot reorganize itself, then the conclusion that Driesch and Morgan drew in regard to the protoplasm will hold, provided, as seems to be the case, the smaller blastomere of the first two is large enough to produce the typical structures. The main point is this: If the protoplasm readjusts itself after the operation, so that the piece divides as a whole, a complete embryo develops; if, however, the protoplasm does not readjust itself, and the piece divides as a part, an incomplete embryo is formed. Since in both cases the same nucleus is present, and since the difference is obviously connected with a change in the protoplasm, it seems much more probable that the phenomenon of whole and half development is connected with the protoplasm and not with the nucleus.
The hypothesis that Pflüger, Hertwig, and Driesch have adopted, namely, that the cleavage divides the egg into potentially equal parts, stands in sharp contrast to the Roux-Weismann conception of development. There are two ideas in the former view which should be kept, I think, clearly apart: the first is, that the blastomeres are potentially equal (isotropous), because they are exactly alike; the second is, that despite the differences that may exist amongst them they are still potentially able to do the same thing, i.e. they are totipotent. The former alternative is that adopted by Pflüger, Hertwig, and Driesch; the latter view, to which Driesch seems more inclined in his later writings, is the one that I should prefer.[126] The first four blastomeres of the sea-urchin’s egg appear to be exactly alike, and we find that each can make a whole embryo. If we assume, however, that despite their likeness and their totipotence they are different in so far as there is present in the protoplasm a bilateral structure, we are nearer, in my opinion, to the truth; for, unless we assume the bilateral structure to be determined later by some external factor, of which there is no evidence, we must suppose that after fertilization, at least, there must be a bilateral structure to the protoplasm, and this view is borne out in one sense by the subsequent mode of cleavage of the blastomeres if they are separated. Whether this bilaterality of the fertilized egg leads to the bilaterality of the cleavage is, however, a different question. In some cases this appears to be the case, in others it is clearly not the case, and we must suppose that some other condition determines the bilaterality of the later stages than that which influences the cleavage. Many facts of experimental embryology and of regeneration show, moreover, that a new bilateral structure may be readily assumed by pieces that have lost their connection with the rest of the organism.
After the third division of the egg of the sea-urchin, four of the blastomeres are somewhat different, so far at least as the material of which they are made up is concerned, from the other four; yet any one of the eight blastomeres, or groups of blastomeres, can produce a whole embryo. The same statement can be made for much later stages, since it has been found that fragments from any part of the blastula wall can give rise to whole embryos, and we may safely attribute this property to all the cells, although on account of the size of the cells of later stages they cannot individually produce a whole embryo, but each can produce any part of an embryo, which amounts to the same thing. If we assume that all of these cells are exactly alike, as Hertwig has done, we fail to see how the next stage in the development could take place, unless some external factor could act in such a way as to change the different parts of the egg. We have, however, no reason to suppose that all the cells are alike because they are all potentially equal. Even pieces of an adult animal—of hydra or of stentor, for example—can produce new whole organisms, although we must suppose these pieces to be at first as unlike as are the parts of the body from which they arise. Moreover, we do not know of a single egg or embryo in which we cannot readily detect differences in different parts of the protoplasm.
Can these gross differences, that we can see, in the materials of the egg explain the different development of the parts of the egg? It can be shown, I think, that they do not necessarily determine the result. If we cut in two a blastula, so that one piece contains only the cells from the animal half and the other piece cells from the vegetative half, each produces a whole embryo; yet the one half lacked just those parts which by hypothesis were supposed to determine the gastrulation of the other half. If we suppose that the materials or structures that are characteristic of the vegetative half are gradually distributed from the vegetative to the animal pole in decreasing amounts, then any piece of the egg will contain more of these things at one pole than at the other. If, then, it could be shown that the gastrulation depends on the relative amounts of these materials in the different parts of the blastula, the difficulty met with in the former view disappears in part. I say in part, because the relative amount of materials that produces the results implies a connecting substratum that is acted upon and determines the result. Even if we suppose that this polar distribution of material could account for the polar invagination, we should still be at a loss to account for the origin of the bilateral symmetry. In many eggs there is no evidence of a bilateral distribution of the material, although in some few cases there may be, so far as the form is concerned, a plane of bilateral symmetry. But even if it is supposed to be present in all eggs, and to coincide with the first plane of cleavage (or with any other cleavage plane), we still could not explain the bilateral symmetry of the one-half and one-fourth whole embryos that come from the corresponding isolated blastomeres. If a preëxisting bilateral plane exists in the egg, it must be reëstablished in some way in the isolated blastomere and in pieces of the blastula wall. In the latter case this could scarcely be brought about by a redistribution of the gross contents of the piece, since the presence of cell walls would interfere with such a process.