88 CLEAVAGE AND DIFFERENTIATION 



p. 68, and Chap, ix) ; accordingly the latter can survive absence of 

 a nucleus during the time required for the zygote nucleus to divide 

 four times, while the former loses its capacity for complete de- 

 velopment if it has remained enucleate for a longer time than that 

 required for three divisions of the zygote nucleus.^ 



The equivalence of nuclei at later stages of cleavage has been 

 established from experiments conducted on the eggs of insects. The 

 egg of the dragon-fly Platycnemis is an elongated structure in which 

 the nucleus is central and divides several times before its products 

 of division reach the surface of the egg and the cytoplasm is par- 

 titioned off into separate blastomeres. By focussing a pencil of 

 ultra-violet rays on a nucleus at the 2-nucleus stage (corresponding, 

 of course, to the 2-cell stage of forms with ordinary cleavage) it is 

 possible to kill it. But the remaining nucleus and its products of 

 division are sufficient to allow a normal embryo to be formed. 



The insect egg is further peculiar in that it possesses near its hind 

 end a region which is essential for the subsequent differentiation 

 of the embryo (see Chap. vi). But the activities of this region are 

 not manifested unless some of the nuclei which have resulted from 

 cleavage migrate into it. This "population" of the hinder end, and 

 indeed of all the surface of the egg, by nuclei, normally takes place 

 after the 5th cleavage, corresponding to the 3 2-cell stage. Again, 

 by means of ultra-violet rays, it is possible to affect a zone of cyto- 

 plasm of the egg in such a way that the products of division of the 

 nuclei are delayed in passing through it, and instead of receiving 

 nuclei after the fifth cleavage, the hinder end only receives them 

 after the eighth cleavage, i.e. at the 256-cell stage. Nevertheless, 

 these nuclei are adequate to activate the region in question, and 

 normal embryos are produced. Here, then, is evidence that the 

 division of the nuclei is qualitatively equal as far as the 256-cell 

 stage.^ 



^ We have already noted that an isolated ventral half, since it does not contain 

 any of the organiser-region, is incapable of development beyond a stage roughly 

 equivalent to the late blastula. It might be supposed therefore that nuclei which 

 had been restricted to a ventral half had been in some way affected so as to be 

 unable to promote full development on passage into a dorsal half. There is, 

 however, no positive evidence for such a possibility, while the greater suscepti- 

 bility of the dorsal half of the egg is an established fact (Spemann, 1901B, 1902 

 1903, 1914, 1928; Ruud and Spemann, 1923). 



2 Seidel, 1932. 



