128 DEVELOPMENT OF THE FROG'S EGG [Cn. XII 



cate that a certain formal self-differentiation of many parts 

 of the segmenting egg has taken place. On the other hand, 

 the fact of postgeneration shows that in each of the first 

 blastomeres a power sufficient to complete the whole must 

 also be potentially present. In order to awaken this potential 

 power of a blastomere, a disturbance in the develoiDment must 

 occur. This latent activity may be only slowly awakened in 

 the development, sometimes sooner, sometimes later. 



We have, therefore, to distinguish two sorts of development, 

 — the normal "direct" development, and an ''indirect" post- 

 generative (or regenerative) development. The first or direct 

 is the result of the self-differentiation of the early blastomeres, 

 and of the complexity of their derivatives. T'he second or 

 indirect is the result of a profound correlation which adds to 

 an imperfect whole the lacking parts. Should the postgenera- 

 tion set in immediately after the isolation of the blastomere 

 and so convert the blastomere at once iiito an actual whole, 

 then we should not have found out that each blastomere is 

 really a self-differentiating cell, but we should have erroneously 

 concluded that the first (four) cleavage-cells are qualitatively 

 equivalent. Into this error Roux believed Driesch and Hert- 

 wig to have fallen. In the frog, ascidian, and ctenophor each 

 of the first blastomeres is specifically different from the others, 

 but in respect to postgeneration we find that each blastomere 

 has the same potentiality, and each is in reality totipotent. 

 The '•'•idioplasm" in direct (i.e. normal) development, called 

 into activity by the process of fertilization, is divided qualita- 

 tively and unequally during the cleavage, while the material 

 which may later serve for postgeneration and regeneration 

 (which is not active during the normal development) is always 

 equally or quantitatively divided. 



According to Roux, the nucleus represents the controlling 

 power of the cell, but the protoplasm acts as a stimulus to 

 the nucleus and hence may indirectly regulate the process of 

 cleavage. " In the telolecithal frog's egg the position of the 

 food-substances and formative substances stands in strict causal 

 relation to the position of the main axes of the embryo." The 

 nuclei of eggs in which the normal arrangement of the contents 

 has been disturbed will be influenced during the first cleavage- 



