CHROMOSOMES 255 



may be specially concerned in the development of 

 particular parts. 



Sutton and others have shown that the different 

 cl romosomes contained in the same nucleus of a partic- 

 ular animal may be of different shapes and sizes, so 

 tl&t each is individually recognisable. It is thus pos- 

 sible to demonstrate that an identically similar set of 

 chromosomes appear at each of several successive cell 

 divisions. In this way additional evidence is afforded 

 of the individual persistence of the chromosomes and 

 of their separate identity. 



We have already pointed out how, in the process of 

 fertilization, the two conjugating germ-cells, as well 

 as the nuclei which they contain, become completely 

 fused together to form a single cell containing only one 

 nucleus. It might have been expected that the sepa- 

 rate chromosomes contained in the conjugating nuclei 

 would also fuse together in pairs during this process, but 

 this is not the case. The paternal and maternal chromo- 

 somes remain separate, so that the nucleus of the zygote 

 contains twice as many chromosomes as does that of 

 either of the gametes by the fusion of which it arose. 

 This double number of chromosomes reappears at every 

 cell division during the embryonic history of the 

 zygote, and thus the fact is accounted for that [with 

 certain exceptions to be mentioned later (p. 272)] 

 the number of chromosomes in a somatic nucleus is 

 always even. Thus we see that the chromosomes 

 derived from the two parents are present side by side in 

 the nuclei of the offspring, and reproduce themselves by 

 bipartition at every nuclear division which takes place 



