258 RE GENERA T1ON 



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 con- 

 trary 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 or- 

 ganism, and that the protoplasm alone undergoes a progressive change. 



The central problem for embryology is trie 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 ques- 

 tion 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. Be- 

 cause 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 



