The experimental work that PflÜger carried out in 1883 on the effect of gravity on the cleavage of the frog’s egg, and the conclusions that he drew from his experiments, mark the starting-point for the modern study of experimental embryology.[124] We can trace the influence of PflÜger’s results through most of the more recent work, and one of the conclusions reached by PflÜger, namely, that the material of the egg may be divided by the cleavage planes in any way whatsoever without thereby altering the position of the embryo on the egg, is, I think, one of the most important results that has yet been reached in connection with the experimental work on the egg. PflÜger’s analysis of the factors that direct the development has also an important bearing on the interpretation of the development of a whole embryo from a part of an egg.

PflÜger found that in whatever position the frog’s egg is turned before it begins to divide, the first two planes come in vertically, and the third horizontally, and that later the smallest cells are always formed in the upper hemisphere. He concluded, therefore, that gravity has some sort of influence in determining the position of the planes of cleavage. PflÜger observed that the position of the median plane of the body of embryos that have developed from eggs turned into unusual positions does not, as a rule, correspond to the plane of the first cleavage, but that the embryo generally lies on that meridian of the egg that passes through the primary egg axis and the highest point of the egg in its new position. Since any meridian may happen to be placed uppermost, the embryo may, therefore, develop upon any one of the primary meridians, and hence the material must be isotropous around the primary axis. Furthermore, since the embryo appears always below the middle of the egg, in whatever position the egg may lie, we must conclude that in each meridian the material is also isotropic.

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.

This analysis shows, I think, that the transformation of a piece into a new whole really involves a change in the fundamental structure itself. There cannot be much doubt that both the polarity and the bilaterality of the egg, or of a piece of the egg, belong fundamentally to the same class of phenomena, and we are forced to the supposition that they are inherent peculiarities of the living substance. Driesch thought at one time that it is only necessary to suppose that the protoplasm, and every part of it, possesses a primary polarity, and that some inequality in the material might determine the plane of bilaterality; but later he thought it necessary to assume also the presence of a bilateral structure in the protoplasm, and in all parts of it. This assumption of every part having a polar and a bilateral structure, and the polarity and bilaterality of the whole being the sum total of those of all its parts, is, I think, insufficient to meet the situation. If, for example, the first plane of cleavage coincides with the median plane of the body, the right blastomere has a structure that leads to the formation of the right side of the body, and similarly for the left blastomere. If the two blastomeres are separated, and each gives rise to a whole embryo with a new plane of bilateral symmetry, we must suppose that a new bilaterality has been produced. It does not make the problem any simpler to assume, as Driesch has done, that this is brought about by the elements rearranging themselves bilaterally on each side of a new plane that passes through the middle of the isolated blastomere, for what we need to have explained is what determines the new median plane. It seems to me that the problem is not any simpler, if we assume the polarity and bilaterality to be the property of a large number of elements, as Driesch has done, than if we assume at once the polarity and bilaterality as characteristic of the whole egg. The difficulty of understanding how a new bilaterality can be induced in a piece of the whole is as great on the one assumption as on the other. Not only is it, I think, a much simpler idea to suppose the structure is something pertaining to the whole and is not the sum total of smaller wholes, but the idea is more in accord with other phenomena.

We meet here, I think, with precisely the same problem that we meet with in the regeneration of parts of adult animals. If a planarian is cut in two lengthwise, along the middle line, each half produces new tissue at the cut-side, out of which the missing half is formed. In this case the old median plane remains, more or less, as the median plane of the new worm, i.e. the structure of the new part is built up on that of the old. Very much the same result follows when the worm is cut longitudinally into two unequal parts. The larger piece retains its old plane of symmetry and adds to the cut-edge a new part that completes the symmetry. The smaller piece also builds up new material along the cut-edge, and a new plane of symmetry is formed between the old and the new parts. Here, also, a median plane is established at the edge of the old material, but in this case the material lay to one side of the old middle line, and this involves the changing over to a large extent of the old material, so that it fits in with the new structures of the new median plane.

In those forms in which the readjustment takes place entirely in the old part, the change of conditions is more difficult to interpret. In some respects hydra gives us an intermediate condition, but since it is a radially symmetrical instead of bilaterally symmetrical form, the transformation is not so obvious. If a cylindrical piece is cut from the body, and is then cut lengthwise into two half-cylinders, each closes in and makes a cylinder of smaller diameter. A little new tissue may appear along the fused edges, but the missing half is not replaced, and a new hydra with a body of half size is formed from the piece. It is to all appearances a radially symmetrical form, and we must think, in this case, of the new axis of symmetry as having shifted to the middle of the piece. As yet no similar experiments have been made on a bilateral animal that regenerates by morphallaxis, so that we have nothing to appeal to for comparison with the bilateral egg, but the results, just described for the planarian and for hydra, indicate how a change might take place in pieces of adult animals that would lead to the formation in them of a new symmetrical structure. If we imagine a case of this sort, and suppose that after separating a piece from the side the cut-edge closed in and the piece assumed a symmetrical form, it is conceivable that a new plane of bilateral symmetry might soon appear in the middle of the piece owing to the symmetrical form of the piece; or the new plane of symmetry might slowly shift from the cut-edge toward the middle of the piece, after reaching which the balance or equilibrium would be attained. This statement, it must be confessed, is little more than a supposition, and rests on the unproven assumption that the internal symmetry may develop in response to a symmetrical change in shape of the piece as a whole, which is partly the outcome of purely physical factors. At present, however, I see no other probable inference from the facts.

If we suppose a bilateral structure is present in the fertilized egg, and that it corresponds to the first plane of cleavage, a change of the sort that we have just sketched above may be supposed to take place when the blastomeres are separated. The stimulus is found in the new spherical form assumed by the isolated blastomere, and we may imagine the change to take place, in the way indicated, by virtue of the old bilaterality that is present, the change beginning at the side originally in contact with the other half.

There are several facts which seem to indicate that a change in the axial relations of the egg is very easily brought about before any definite organs have appeared. The fact that the point of entrance of the spermatozoon in the egg of the frog[127] and of the sea-urchin[128] may determine the first plane of cleavage points to this conclusion. The fact that, in the frog, and also in the triton, the median plane of the embryo corresponds sometimes to the first, sometimes to the second plane of cleavage, and sometimes to neither one, shows that the bilaterality of the embryo-structure may or may not coincide with the plane of cleavage. In the fish also there seems to be no correspondence between the planes of cleavage and those of the embryo, so that different factors may determine the two. We should not be justified in concluding from this evidence that a bilateral structure is absent, but rather that it is of such a sort as to be independent of the cleavage, and that it can be also easily changed. It is probable that the kind of organization that we must suppose to exist in the egg is of a very simple sort, and capable of easy readjustment. There is certainly no evidence in favor of the view that the organization of the egg need be anything like the organization of the embryo that comes from the egg, although the organization of the egg may be perfectly definite in its character. Until we know more of the nature of this organization, it is useless to speculate further as to how it can be altered.

Another question of much importance in connection with our present topic is the part played by the individual cells in the early development of the whole egg, or of any part of the egg. Hertwig (’93) thinks that the development is brought about by the action of the individual cells on each other. Driesch, when he states that the fate of a blastomere is a function of its position in the whole, does not commit himself definitely one way or the other so far as the cell as a unit is concerned. Whitman and others have urged the insufficiency of the cell theory, and think that cell boundaries play no important part in the development, but that the embryo develops as a whole. This has seemed to me to be the more probable view in the light of certain results of experimental embryology. Driesch, in later papers, has also opposed Hertwig’s idea, and Wilson in his book on The Cell has also, to a certain extent, adopted this point of view. The formation of a typical larva in the sea-urchin and in amphioxus out of one-half or one-fourth the whole number of cells demonstrates, I think, the insufficiency of the cell-unit hypothesis. The discovery of continuous protoplasmic connections between neighboring cells, and the formation of new protoplasmic connections between all regions, as found by Mrs. G. F. Andrews,[129] gives us a basis of fact on which to rest the hypothesis of the embryo being a whole structure. This view meets with no great difficulty on the grounds that the nuclei are distinct centres of metabolic activity, for we know at present so little of what sort of action takes place between the nucleus and the protoplasm that we cannot rest our argument on any demonstrable relation.

The discovery that pieces below a certain minimum size are incapable of producing a whole organism is of capital importance. It has been pointed out that pieces of the egg of the sea-urchin less than one-sixteenth of the whole do not produce even the gastrula stage. In amphioxus the one-eighth blastomere seems to be near the lower limit of development. It has also been found that there is a lower limit for pieces of adult organisms below which they do not regenerate. This has been shown for hydra, tubularia, planarians, and stentor, and is probably true for all forms. This result is especially interesting in those cases in which the parts contain all the elements necessary to produce a new organism, and come from parts of the body that are totipotent in these respects. It seems certain that the lack of power of development in these cases is due entirely to the smallness of the piece. We can express the idea in another way by stating that a certain volume is necessary in order that a piece may produce the typical organization. This conclusion is important as showing that the organization is something enormously large as compared with the size of the chemical or physical molecules, and even of the crystal molecule. The size of a piece that is at the lower limit of organization is also very much larger than the smallest cells of which the embryo is made up, and this relation is a point in favor of the view that the organization is not simply the resultant of the interaction of the cells, but is something much larger than these cells; and we may even go further, I think, and add that it dominates the cells rather than is controlled by them.

In the light of the questions discussed in the preceding pages, we may now attempt to follow out in a more connected way some of the modern views and hypotheses dealing with the problem of development.

Hertwig, as we have seen, has opposed the Roux-Weismann hypothesis, and has formulated a view of his own. According to Hertwig, the cleavage divides the egg into equivalent parts,—an idea very similar to that of PflÜger. The cells he regards as units, and the development as the result of the interaction of the cells,—a process that in a way Roux had also assumed to take place between the different parts of the later embryo. Thus, while Hertwig’s hypothesis contains little that is really new, it has selected portions from several already existing hypotheses, and united them into a consistent whole. It has been objected to Hertwig’s view that the interaction of equivalent cells could never account for the introduction of new processes in the development; but if we grant that the cells are never entirely equivalent, whatever their potence may be, this objection can, I think, be met. Hertwig’s chief service has been his destructive criticism of the Roux-Weismann idea of qualitative nuclear division.

Hertwig maintains that each stage in the development is the cause of the next stage, and states that a description of the series of stages through which the embryo passes gives a causal knowledge of the phenomena of development. He claims that beyond this descriptive knowledge we cannot hope to penetrate. Both Roux and Driesch have taken issue with Hertwig, and have pointed out that while each stage in the development contains within itself the causes of the succeeding stage, yet we gain no idea as to these causes from a simple description of two consecutive stages themselves. To state that the fertilized egg is the cause of the cleavage gives us no idea of what sort of a process the cleavage is, or how it arises, or what determines the sequence of the divisions, etc. The blastula, for instance, contains the factors that produce the gastrula; but to state that, in a physical sense, the blastula is the cause of the gastrula is an erroneous interpretation of what is meant by causal knowledge. If Hertwig’s idea were correct, there would be as many causes in each embryo as there are stages in its development, and as many causes in the whole range of embryology as there are forms that develop multiplied by the number of stages in each embryo. What we should seek to discover is the particular cause that brings about each kind of process. If we could discover the cause in one single case, it is highly probable that it would be found to extend to a large number of other cases.

Driesch formulated an hypothesis of development in his Analytische Theorie, but has modified and changed it in several later contributions. It is difficult to give in a few words the subtile analysis which Driesch has made of the phenomena of development. His analytical theory rests on the dictum that the prospective value of each blastomere is a function of its position in the whole. By “function” is meant “a relation of dependence of a general unknown kind.” This idea is connected with the following one, viz. that any blastomere could be interchanged with any other one without altering the end-result. A few elementary processes are supposed to be “given” in the structure, or in the composition of the egg. Each elementary process is the outcome of a cause, and each elementary process must release the succeeding causes,—i.e. if the organization of the phase A is present, one of the causes of the next phase B is also then present. The first elementary process is the cleavage, that is initiated (“ausgelÖst”) by the fertilization. After a fixed number of divisions has taken place, the cleavage process comes to an end. It has led to the production of a number of cells having similar nuclei but having a different plasma structure, and the result is the blastula stage. Organs whose formation starts from the blastula stage are called primary organs; the archenteron, the mesenchyme, the ciliated band, and the mouth of the sea-urchin embryo belong to this class. Secondary organs are those that arise from the primary ones, as the coelom sacs, for instance, in the sea-urchin embryo. The primary organs are started by the setting free (“AuslÖsung”) of a new elementary process in the blastula, and later the secondary organs are started by new elementary processes that arise in the gastrula, which cannot appear until the gastrula stage itself is present as a starting-point. In other words, the elementary processes that are “given” in the egg can only come into action, or be set free after a certain stage has come into existence. This means that we must think of each organ that responds to a stimulus as having the possibility of receiving that stimulus, and also of answering to it. Even in inorganic nature every reaction must depend on a specific receptiveness and a specific answer. Driesch supposes that the receptivity is in the protoplasm, and the power to respond is in the nucleus of each cell. In this way he attempts to meet the difficulty that the nucleus is, in every cell, the bearer of the totality of all the “Anlagen”; but inasmuch as it is surrounded by a specific plasma, it is in a position to receive only certain stimuli, and can therefore only respond to certain causes.

In the specific nature of the cytoplasm of the cell lies the prospective potence of every organ, and the possibilities of each cell are limited by its plasma; the cell becomes more and more limited as development proceeds. It may be said, therefore, that in the course of development the cells become actually limited in their possibilities, although they may still retain within themselves, in the nucleus, the potentialities of the entire organism.

In the course of development each causal reaction brings about not only chemically specific differences, and thereby makes possible the introduction of new elementary processes, but the reaction also brings about by this very means a lessening of the possibilities of the cell, because each cell will now only respond to a more limited set of causes. We may say that the elementary process is not only the cause of the next change, but by virtue of its specific nature it is the beginning stage of the future reactions. Development proceeds from a few prearranged conditions, that are given in the structure of the egg, and these conditions, by reacting on each other, produce new conditions, and these may in turn react on the first ones, etc. With every effect there is at the same time a new cause, and the possibility of a new specific action, i.e. the development of a specific receiving station for stimuli. In this way there develops from the simple conditions existing in the egg the complicated form of the embryo.

In this brief summary of some of the essential features of Driesch’s hypothesis, I have omitted some parts that seem to me to go beyond the legitimate field of a scientific hypothesis,—such, for instance, as the causal harmony of the reactions; and other parts have been omitted because they are improbable in the light of more recent work. It would not be difficult to show that many difficulties beset each stage of the argument, or to show how slender a basis of fact there is to support some of the hypotheses. In fact, Driesch himself has modified very greatly some of the views of his Analytische Theorie in his later writings. The merits of the analysis should not be overlooked, however, since it is one of the first philosophical attempts to show how, in the light of recent discoveries, the process of epigenetic development may receive a causal interpretation. Even if the argument should break down, the hypothesis will remain an interesting contribution, opening the way to newer points of view in regard to the process of development. In later papers, especially in those dealing with the localization of morphogenetic processes, Driesch attempts to show that certain experimental results demonstrate that there is a vitalistic principle at work in the development of the organism from the egg, as well as in the process of regeneration. He bases his argument on the results of the experiment in which the gastrula of the sea-urchin egg is cut in two, as described already on page 234. The archenteron has not, at the time of the experiment, subdivided itself into its three characteristic parts. The posterior piece, that contains the posterior part of the archenteron (the anterior part having been removed with the anterior piece), produces a new whole embryo of smaller size, in which the archenteron is subdivided into three parts, that are in the same proportion to each other and to the whole embryo as are the same divisions of the normal archenteron. This proportionate formation of the parts of the archenteron on a smaller scale cannot, Driesch claims, be accounted for on any known chemical or physical principle. There must be, therefore, a different sort of principle involved, and this Driesch calls the vitalistic principle.

It may be pointed out that this illustration that Driesch has selected is only an example of all proportionate development, which many observers have described as taking place in pieces of embryos. It is only a striking case of what has been also known in many cases of regeneration, of small pieces producing whole structures, and there is nothing new or startling in this demonstration of a vitalistic principle. The fact may be stated in another way, viz. that the proportionate development of an organ is, within certain limits, self-determining, or is self-determined by its size. The vitalistic principle that Driesch sees demonstrated in these results is the now familiar process of a smaller piece producing the typical structure on a smaller scale; a phenomenon that a number of other writers had already called attention to as one of the most remarkable phenomena connected with the regeneration of pieces of an adult organism, or of an egg.

It is something of this same sort that the older zoologists must have had in mind when they spoke of “formative forces” as peculiar to living things. The use of the word “force” in this connection has often been objected to, and not without justification; since it seems to imply that the action is of the sort for which the physicist uses the word “force.” The fundamental question turns upon whether the development of a specific form is the outcome of one or more “forces,” or whether it is a phenomenon belonging to an entirely different category from anything known to the chemist and the physicist. If we state that it is the property of each kind of living substance to assume under certain conditions a more or less constant specific form, we only restate the result without referring the process to any better-known group of phenomena. If we attempt to go beyond this, and speculate as to the principles involved, we have very little to guide us. We can, however, state with some assurance that at present we cannot see how any known principles of chemistry or of physics can explain the development of a definite form by the organism or by a piece of the organism. Indeed, we may even go farther and claim that it appears to be a phenomenon entirely beyond the scope of legitimate explanation, just as are many physical and chemical phenomena themselves, even those of the simplest sort. To call this a vitalistic principle is, I think, misleading. We can do nothing more than claim to have discovered something that is present in living things which we cannot explain and perhaps cannot even hope to explain by known physical laws.

Wilson (’94) has also rejected Roux’s hypothesis of qualitative nuclear division, and adopts the view of the totipotence of the early blastomeres. He has also advanced the view that there is during development a progressive differentiation of the cells. In a later contribution (’96), he accepts “the view of Hertwig and of Driesch that the various degrees of partial development beginning with the echinoderm egg and culminating in the gasteropod may be due to varying conditions of the egg cytoplasm in the different forms.” Wilson points out that the series of forms represented at one end by amphioxus and at the other end by the ctenophore and the gasteropod may be brought under a common point of view, “for it is certain that development must be fundamentally of the same nature throughout the series, and the differences must be of secondary moment.”

If we reject, as several students of experimental embryology and of regeneration have done, the Roux-Weismann idea of the existence of pre-formed germs in the nucleus, and also the idea of Hertwig of the equivalency of the first-formed blastomeres, and Driesch’s vitalistic principle, what position can we take in regard to the problem of development? We may at least attempt to formulate our present position.

There must be assumed to exist in the egg an organization of such a kind that it can be divided and subdivided during the cleavage without thereby losing its primary character. The refusion of the cells after each division by means of protoplasmic connections indicates how this may be possible. The organization must be thought to be of such a kind that the factors determining the cleavage may be different from those that determine the median plane of the body. This is demonstrated by PflÜger’s experiment in which the position of the cleavage planes is changed, but the embryo appears in relation to the primary meridians. The first-formed blastomeres, that result from the division of the egg, do not seem to be strictly equivalent, but they appear to be in most cases, at least, totipotent. The characteristics of each part of the protoplasm may be a factor in determining what sort of structure may come from that part of the egg, but back of this lies the fundamental character of the protoplasm itself, that determines what each part, in its relation to the whole, can do. The division of the nucleus appears to be in all cases an exact quantitative division, and there is some evidence to show that the early nuclei are all equivalent,—or at least totipotent. The division of the protoplasm is often into unlike parts, and the kind of cytoplasm contained in a part may or may not limit the potencies of each part.

One of the most important facts in connection with the organization is that a part, if separated from the rest, may become a new whole, and this appears to be a fundamental peculiarity of living things. Analogies can be found, perhaps, in inorganic phenomena, as for instance a storm dividing into two or more parts and each developing a new storm centre of its own, or when a suspended drop is divided and each half becomes a new sphere; but these comparisons lack some of the essential features of the organic phenomenon.

A progressive change takes place as development proceeds, so that a stage once passed through is not repeated if a part is separated from the rest, as illustrated by Driesch’s experiments with the blastula and gastrula of the sea-urchin and starfish, and by the method of development of pieces of the adult, that do not pass through the embryonic stages. As the protoplasm changes new conditions may arise, either because the protoplasm in its new form can be acted upon by those internal or external conditions to which it did not respond at first, as Driesch has supposed, or, as I think equally probable, because the series of reactions that have begun with the first step in the development work themselves out in the same way that a chemical reaction once started may pass through a long series of stages depending upon the nature of the substance. The difference between these views lies in this, that the former supposes latent substances, or elementary processes or forces, whatever they may be called, to be present in the egg and to act when a medium that responds to them has come into existence; the other idea supposes that the whole process is started with the first change and once set going is of such a kind as to continue to an end through a regular series of stages. Both views are suppositions, and, it may be, reduce themselves ultimately to the same thing.

On any theory of development, the nucleus cannot be left out of account, since the evidence that we now possess shows that through the nucleus even the most trivial peculiarity of one parent, and probably of both, may be transmitted. This has led a number of zoologists to look upon the nucleus as a body containing specific elements corresponding to those of the individual from which the nucleus has come, but inheritance through the nucleus is no more a demonstration of the existence of pre-formed elements of the male than are the general facts of embryology a demonstration of pre-formation. All we can legitimately conclude is that the substance of the nucleus is of such a sort that it acts on the cytoplasm in a definite way, and determines, in part at least, its differentiation. There has been steadily accumulating evidence to show that during development there is an interchange of material between the nucleus and the protoplasm, and it is not going far afield to conclude that the character of both nucleus and protoplasm is altered by the interchange in material. If this is admitted it is no more remarkable that a hybrid is midway between its parents than that a parthenogenetic egg produces a form like that of the individual from which it has come.

Several writers, as we have seen, have adopted the view that the nuclei are storehouses of the undifferentiated germ plasm, and retain everywhere the sum total of the “Anlagen” of the egg nucleus. I do not know of any evidence that demonstrates that the nucleus is less modified in these regards than is the rest of the cell. On the contrary it seems to me that a fair case might be established in favor of the view that the nucleus and the cytoplasm cannot be contrasted in this way, and that a change in the cytoplasm may also involve a change in the nucleus.

The phenomena of regeneration show over and over again that differentiated cells may change into structures entirely different from what they have been, as illustrated in the development of the lens from the edge of the iris, and in the production of a new hydra, or tubularian, from a piece of an old one. It is, I think, an arbitrary assumption to suppose that this is brought about by a reserve stuff in the nucleus, for the production of new eggs and spermatozoa in the animal, from cells that have themselves passed through most of the early embryonic changes and have been parts of embryonic organs, shows that although the protoplasm may change throughout these stages, it may still come back to the starting-point, and there is nothing to show that this return is brought about by the nucleus. I cannot but think that Driesch was prejudiced by current opinion, when he adopted the view, as one of the foundations of his analytical theory, that the nucleus contains all the “Anlagen” of the whole organism, and that the protoplasm alone undergoes a progressive change.

The central problem for embryology is the determination of what is the cause or causes of differentiation. Our analysis leads us to answer that it is the outcome of the organization; but what is the organization? This it must be admitted is a question that we cannot answer. Looked at in this way the problem of development seems an insoluble riddle; but this may be because we have asked a question that we have no right to expect to be answered. If the physicist were asked what is gravity he could give no answer, but nevertheless one of the greatest discoveries of physics is the law of gravitation. If we could answer the question of what the organization is to which we attribute the fundamental phenomenon of development, there would perhaps be nothing further left to find out in the development of animals. Fortunately there is a different and safer point of view. There are other questions to which we can expect an answer. Because the physicist cannot tell what gravity is, he neither rejects the term nor despairs of obtaining a knowledge of how it acts. If our analysis of the problem of development leads us to the idea of an organization existing in the egg, our next problem is to discover how it acts during development. Most of the results described in several of the preceding chapters have taught us something of how the organization behaves. We have found that it can be affected by external circumstances, even to such an extent that its polarity may be reversed. We have seen that if an organized structure is broken up into pieces, each piece may reorganize itself into a new whole. The most familiar, and at the same time the most difficult thing to understand, is that the organization is of such a kind that it has the property of passing through a definite series of stages leading to a typical result, and having reached its goal of throwing off organized bodies, or germ cells, that begin once more at the starting-point and pass through the same cycle. The action of the organism is sometimes compared to that of a machine, but we do not know of any machine that has the property of reproducing itself by means of parts thrown off from itself.

These are some of the most characteristic phenomena exhibited by the organization. In the final chapter some of the questions that have been suggested in connection with the method of action of the organization will be further discussed.