TEE CELLULAR BASIS OF HEREDITY 



119 



At the first cleavage of the egg each of these substances is divided 

 into right and left halves (Fig. 29, 5). The second cleavage cuts off 

 two anterior cells containing the gray crescent from two posterior ones 

 containing the yellow crescent (Fig. 29, 6 and Fig. 30, 1). The third 

 cleavage separates the colorless protoplasm in the upper hemisphere 

 from the slate-blue in the lower (Fig. 30, 2). And at every successive 

 cleavage the cytoplasmic substances are segregated and isolated in par- 

 ticular cells, — and in this way the cytoplasm of the different cells comes 

 to be unlike (Figs. 30 and 31). When once partition walls have been 



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Fig. 30. Cleavage of the Egg of Stycla, showing distribution of the yellow 

 protoplasm (stippled) and of the clear and gray protoplasm to the various cells, each 

 of which bears a definite letter and number. 



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 Amphioxiis and of the frog, though the segrega- 



