THE CELLULAR BASIS 125 



At the first cleavage of the egg each of these 

 substances is divided into right and left halves 

 ( Fig. 27, 5 ) . The second cleavage cuts off two 

 anterior cells containing the gray crescent 

 from two posterior ones containing the yellow 

 crescent (Fig. 27, 6 and Fig. 28, 1). The 

 third cleavage separates the colorless proto- 

 plasm in the upper hemisphere from the slate 

 blue in the lower (Fig. 28, 2). And at every 

 successive cleavage the cytoplasmic substances 

 are segregated and isolated in particular cells, 

 and in this way the cytoplasm of the differ- 

 ent cells comes to be unlike (Figs. 28 and 29) . 

 When once partition walls have been formed 

 between cells the substances in the different 

 cells are permanently separated so that they 

 can no longer commingle. 



What is true of Styela in this regard is 

 equally true of many other ascidians, as well 

 as of Ampliioocus and of the frog (Figs. 6, 9, 

 10), though the segregation of substances and 

 the differentiation of cells are not so evident 

 in the last named animals because these sub- 

 stances are not so strikingly colored. Indeed 

 the segregation and isolation of different 



