CHROMOSOMES 255 
may be specially concerned in the development of 
particular parts. 
Sutton 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. 270 for an exceptional case. 
