THEORIES OF INHERITANCE AND DEVELOPMENT 



ment. And if the germinal localization is not directly determined by 

 the nuclei it must here be determined by a pre-organization of the 

 cytoplasmic substance. How is this result to be reconciled with the 

 experiments on Amphioxus and the echinoderms, and with the more 

 general conclusion that the ultimate determining causes of differentia- 



Fig. 139. Partial development of isolated blastomeres of the gasteropod egg, Ilyanassa. 

 [CRAMFTON.] 



A. Normal 8-cell stage. B. Normal i6-cell stage. C. Half 8-cell stage, from isolated blasto- 

 mere of the 2-cell stage. D. Half 12-cell stage succeeding. E. Two stages in the cleavage of an 

 isolated blastomere of the 4-cell stage; above a one-fourth 8-cell stage, below a one-fourth i6-cell 

 stage. 



tion are to be sought in the nucleus ? The difficulty at once disap- 

 pears when we recall that development arid differentiation do not in 

 any proper sense first begin with the cleavage of the ovum, but long 

 before this, during its ovarian history. The primary differentiations 

 thus established in the cytoplasm form the immediate conditions 

 to which the later development must conform ; and the difference 



