180 HEREDITY AND ENVIRONMENT 



structures which are characteristic of the dif- 

 ferent kinds of tissue cells. We shall here de- 

 scribe only the first and second of these pro- 

 cesses which are of more general interest than 

 the last. 



1. The Formation of Different Substances 

 in Cells. — Embryonic differentiation consists 

 primarily in the formation of different kinds of 

 protoplasm out of the protoplasm of the germ 

 cells. It is plain that different kinds of proto- 

 plasm are present in the two germ cells before 

 they unite in fertilization, but in the course of 

 development the number of these substances 

 and the degree of difference between them 

 greatly increase. 



Actual observation shows that by the inter- 

 action with one another of substances or parts 

 originally present and by their reactions to 

 external stimuli new substances and parts ap- 

 pear which had no previous existence just as 

 new substances result from chemical reactions. 

 This is "creative synthesis" in philosophy, 

 epigenesis in development. Differentiations 

 appear chiefly in the cytoplasm but only as the 

 result of interaction between cytoplasm and 



