140 Heredity and Environment 



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 

 which continue during every cleavage of the egg, lead to the segre- 

 gation of different kinds of plasms in different parts of the egg 

 and to the unequal distribution of these substances to different 

 cells (Figs. 10, 46, 47). 



One of the most striking cases of this is found in the ascidian 

 Styela, in which there are four or five substances in the egg 

 which differ in color, so that their distribution to different regions 

 of the egg and to different cleavage cells may be easily followed, 



