THE CELLULAR BASIS 119 



At the first cleavage of the egg each of these 

 substances is divided into right and left halves 

 ( Fig. 27, 5 ) . The second cleavage uts 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 they permanently separate the 

 substances in the different cells 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 Amphioscus and of the frog, though the 

 segregation of substances and the differentia- 

 tion of cells are not* so evident in the last named 

 animals because these substances are not so 

 strikingly colored. Indeed the segregation 

 and isolation of different protoplasmic sub- 



