126 DEVELOPMENT OF THE FROG'S EGG [Cir. XII 



complexity. The development would then be due to the pro- 

 duction of many parts out of a few primary ones, i.e. the de- 

 velopment is a process of epigenesis. There would thus result 

 an ever-changing interaction of the parts to form the whole, 

 by which means there would be also brought into play a regu- 

 lating influence of the whole back again on the parts, i.e. corre- 

 lation of the parts under the influence of the whole. His's 

 principle of germinal localization would, therefore, have a 

 causal meaning only in so far as it points out the place in the 

 egg where the resulting formation of many-sided changes takes 

 place ; and it would be of only secondary value to be able to 

 refer the place of action of these changes to the undifferen- 

 tiated plasm or to the unfertilized egg. 1 



In conclusion, it should be noted, Roux said, that self-differ- 

 entiation of the parts and dependent differentiation of the parts, 

 i.e. evolution and epigenesis, may be combined in a many-sided 

 activity or union, and it would then be our duty, in order to 

 interpret these problems, to use a double foresight and a double 

 care, to make out the part played by each of these factors in the 

 development. 



Theory of Driesch and of Hertwig of the Equiva- 

 lency of the Early Blastomeres 



Studies on other forms show that great care must be taken 

 in interpreting the results of the experiments on the frog's egg. 

 In 1891 Driesch made a series of most important experiments 

 on the eggs of the sea-urchin. 2 The blastomeres were isolated 

 by shaking them apart, and it was found that although each 

 blastomere segmented as a part, i.e. as if still in contact with 

 the missing half, yet the open side of the blastula closed over 

 very soon, and a gastrula and embryo having the normal form 

 were produced. Driesch concluded from this and similar experi- 

 ments that all the blastomeres are equivalent, and that the posi- 

 tion of each blastomere in the segmenting egg determines in 



1 The formation of two embryos from one egg would take place, on the theory 

 of interaction of the parts, at the time when the median axis of the body is formed. 

 Two such axes would be laid down instead of one. 



2 Fiedler had made, in 1891, a somewhat similar experiment, but it was not 

 carried sufficiently far to be of great value. 



