186 HEREDITY AND ENVIRONMENT 



D. THE MECHANISM OF DEVELOPMENT 



Development consists in the transformation 

 of the oosperm into the adult. What is the 

 mechanism by which this transformation is 

 effected? There is progressive differentiation 

 of the germ into the developed organism but 

 by what process is this differentiation ac- 

 complished? 



Many different processes are concerned in 

 embryonic differentiation. From the stand- 

 point of the cell the most important of these 

 are (1) the formation of different kinds of 

 substances in cells, (2) the localization and 

 isolation of these substances, (3) the trans- 

 formation of these substances into the various 

 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 



