238 RECENT CYTOLOGY 



may be specially concerned in the development of 

 particular parts. 



Button has recently shown that the different chromo- 

 somes contained in the same nucleus of a particular 

 animal may be of different shapes and sizes, so that each 

 is individually recognisable. It was thus possible to 

 demonstrate that an identically similar set of chromo- 

 somes appeared 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 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 

 * See, however, p. 253 for an exceptional case. 



