THE CELLULAR BASIS 123 



an advanced stage of the cleavage (Fig. 26). 

 At the same time that the maternal and 

 paternal chromosomes are being distributed 

 with such precise equality to all the cells of 

 the developing organism the different sub- 

 stances in the cell body outside of the nucleus 

 may be distributed very unequally to the cleav- 

 age 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 segregation of different 

 kinds of plasms in different parts of the egg 

 and to the unequal distribution of these sub- 

 stances to different cells (Figs. 9, 27, 28). 



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 dif- 

 fer in color, so that their distribution to differ- 

 ent regions of the egg and to different cleavage 

 cells may be easily followed, and even photo- 

 graphed, while in the living condition. The 

 peripheral layer of protoplasm is yellow and 

 it gathers at the lower pole of the egg, where 

 the sperm enters, forming a yellow cap (Fig, 

 27, 1). This yellow substance then moves, 



