THE STRUCTURE OF THE FERTILISED EGG 



37 



Fig. 13. "Retarded nucleation" in Triton. Zygote nucleus pushed to 

 one side by the constriction of the egg. (a) first cleavage in the 

 nucleated half; (&) further cleavages in this part, the other half 

 is still un cleaved; (c) passage of one nucleus from the nucleated 

 into the non-nucleated half, beginning of development in the latter; 

 id) the two embryos that developed from the constricted egg; the 

 one produced by the half in which nucleation was retarded is 

 considerably younger, but normally built. After Spemann and 



Fankhauser. 



development of the half in which nucleation was retarded. It 

 is evidently equivalent to the fertilisation nucleus. This proves 

 once again that, at least in the first stages of development, 

 there is no qualitative difference between the division products 

 of the nucleus. 



These and similar experiments prove that Weismann's theory 

 cannot be maintained. Development is not due to a qualitatively 



