140 H'eredity and Environment 



ing" stage similar to that with which the division began, thus 

 completing the "division cycle" of the cell (Fig. 8). 



Identity of Chromosomes. During the whole division cycle it 

 is possible in a few instances to distinguish the chromosomes of 

 the egg from those of the sperm, and in every instance where this 

 can be done it is perfectly clear that these chromosomes do not 

 fuse together nor lose their identity, but that every chromosome 

 splits lengthwise and its halves separate and go into the two 

 daughter cells where they form the daughter nuclei. Each of 

 these cells therefore receives half of its chromosomes from the egg 

 and half from the sperm. Even in cases where the individual 

 chromosomes are lost to view in the daughter nuclei those nuclei 

 are sometimes clearly double, one-half of each having come from 

 the egg chromosomes and the other half from the sperm chromo- 

 somes (Fig. 45). 



At every subsequent cleavage of the egg the chromosomes di- 

 vide in exactly the same way as has been described for the first 

 cleavage. Every cell of the developing animal receives one-half 

 of its chromosomes from the egg and the other half from the 

 sperm, and if the chromosomes of the egg differ in shape or in 

 size from those of the sperm, as is sometimes the case when dif- 

 ferent races or species are crossed, these two groups of chromo- 

 somes may still be distinguished at advanced stages of develop- 

 ment. Where the egg and sperm chromosomes are not thus dis- 

 tinguishable it may still be possible to recognize the half of the 

 nucleus which comes from the egg and the half which comes from 

 the sperm even up to an advanced stage of the cleavage (Fig. 45). 



Distribution of Cytoplasm. At the same time that the mater- 

 nal and paternal chromosomes are being distributed with such 

 precise equality to all the cells of the developing organism the 

 different substances in the cell body outside of the nucleus may 

 be distributed very unequally to the cleavage cells. The move- 

 ments of the cytoplasm of the egg, which began with the flowing 

 of the surface layer to the point of entrance of the sperm, and 



