THE CELLULAR BASIS 117 



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. 



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 dis- 

 tribution to different regions of the egg and 

 to different cleavage cells may be easily fol- 

 lowed, 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 (Fig. 27, 1). 



