FIRST PERIOD OF DEVELOPMENT. 55 



mentation sphere, characteristic independence ; this 

 they now lose, since as epithelial cells they stand 

 in closer dependence on one another. Thus that 

 stage of the blastula is reached which is characterized 

 by an epithelial layer on every side, surrounding a 

 closed inner cavity. This simple epithelial layer forms 

 the substratum for the later processes of development. 

 We shall see how through foldings and growth of 

 this simple layer the most important organs become 

 separated off. During the* whole embryonic develop- 

 ment, all changes from the blastula onwards can 

 be traced to these primitive cell-layers. There never 

 occurs a multiplication of the epithelial layers, or 

 division of the same. 



