THE CELLULAR BASIS OF HEREDITY 1 1 7 



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 may be clearly double, 

 one half of each having come from the egg chromosomes and the other 

 half from the sperm chromosomes (Fig. 26 B). 



At every subsequent cleavage of the egg the chromosomes divide 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 some- 

 times the case when different races or species are crossed, these two 

 groups of chromosomes may still be distinguished at advanced stages 

 of development. Where the egg and sperm chromosomes are not thus 

 distinguishable it may still be possible to recognize the half of the nu- 

 cleus which comes from the egg and the half which comes from the sperm 

 even up to an advanced stage of the cleavage (Eig. 26). 



At the same time that the maternal and paternal chromosomes are 

 being distributed with such precise equality to all the cells of the devel- 

 oping organism, the different substances in the cell body outside of the 

 nucleus may be distributed very unequally to the cleavage cells. The 

 movements of the cytoplasm of the egg which began with the flowing of 

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



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

 in which there are four or five different kinds of substance 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, and even 

 photographed while in the living condition. The peripheral layer of 

 protoplasm is yellow and when it gathers at the lower pole of the egg 

 where the sperm enters it forms a yellow cap. This yellow substance 

 then moves, following the sperm nucleus, up to the equator of the egg 

 on the posterior side and there forms a yellow crescent extending around 

 the posterior side of the egg just below the equator. On the anterior 

 side of the egg a gray crescent is formed in a somewhat similar man- 

 ner and at the lower pole between these two crescents is a slate blue 

 substance, while at the upper pole is an area of colorless protoplasm. 

 The yellow crescent goes into cleavage cells which become muscle and 

 mesoderm, the gray crescent into cells which become nervous system and 

 notochord, the slate-blue substance into endoderm cells and the color- 

 less substance into ectoderm cells. 



