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.
CHAPTER XIII
THEORIES OF REGENERATION
It is significant to find that the theory of pre-formation of the embryo in the egg, that was so very widely held during the seventeenth and eighteenth centuries, and during the first part of the nineteenth century, was at once applied to the explanation of the regeneration of animals when this process became known. Bonnet in 1745 attempted to explain the newly discovered facts in regard to the regeneration of animals by means of the pre-formation theory. Just as the egg was supposed to enclose a pre-formed germ, so he imagined there lay concealed latent germs in the adult animal. At first Bonnet thought that these germs must be whole germs, like those contained in the germ cells of the reproductive organs, and that only as much of any one developed as was needed to replace the missing part. Later, however, he admitted that the germs might be incomplete germs, which are so located in each region that they represent the parts of the body beyond that region. The purpose of these germs is to replace any accidental injuries to the animal. He pointed out that some animals are more subject to injuries than others, and these animals are he thought especially well supplied with germs. Since in some animals the same part may be replaced several times, Bonnet assumed that on each occasion a new germ is awakened. As many sets of germs are present in these animals as the number of times the animal is liable to be injured in the course of its natural life.
Bonnet found that in lumbriculus a new head and a new tail may appear at almost any level, if the worm is cut in two, and, therefore, he supposed, head germs and tail germs are present throughout the worm. But why, if this is so, should a head germ always develop at the anterior end, and a tail germ at the posterior end of a piece cut from the body? Bonnet’s keen mind saw that it was necessary to make a further assumption. He supposed that the fluids of the body that pass forward carry nourishing substances for the head. When the worm is cut in two these substances are stopped at the anterior cut-surface, and there accumulating act on the latent head germ, and nourishing it, cause it to develop. Correspondingly the nourishing substances for the tail flow backward, and accumulating at the posterior cut-surface awaken a tail germ to activity.
The part of the body in which these nourishing substances are supposed to be produced is not specifically stated, but in one passage Bonnet says that the fluids that flow toward the head are there used up in that organ, and we may infer that he held a similar view for the posterior region. He offers no explanation of the cause of the flow of these substances in a given direction, and in this respect his hypothesis lacks support where it is most needed. In fact, it is no more improbable that a head germ should always develop at the anterior end and a tail germ at the posterior end, than that head-forming substances should flow in one direction and tail-forming in another. It is not that it is worth while to object to Bonnet’s hypothesis on the ground that it does not explain everything, but it is worth while to point out that it gives only the appearance of an explanation, and that it begs the whole question by the assumption of particular nourishing fluids flowing in definite directions. So far as the blood is concerned, we know that the different parts of the body take from it those substances or fluids that they make use of, not that special fluids flow to particular regions. It is probable that Bonnet thought of the blood rather than of any other subtler fluids passing through the tissues; and, if so, there is nothing that we know in regard to the behavior of the blood that lends support to Bonnet’s idea.