26 HEREDITY AND ENVIRONMENT 



a constantly increasing number of cells. Dur- 

 ing the earlier divisions there is little or no in- 

 crease in the volume of the egg, consequently 

 successive generations of cells continually 

 grow smaller (Figs. 10, 11). This process 

 is known as the cleavage of the egg, and 

 by it the egg is not only split up into a con- 

 siderable number of small cells, but a much 

 more important result is that the different 

 kinds of protoplasm in the egg become isolated 

 in different cleavage cells, so that these sub- 

 stances can no longer freely commingle. The 

 cleavage cells, in short, come to contain dif- 

 ferent kinds of substance, and thus to differ 

 from one another. The differentiations of the 

 cleavage cells appear much earlier in some 

 forms than in others, but in all cases such 

 differentiations appear during cleavage. 



4. Embryogeny. From this stage onward 

 the course of development differs in different 

 classes of animals to such an extent that it is 

 difficult to formulate any general description 

 which will apply to all of them. Usually the 

 many cleavage cells form a hollow sphere, the 

 blastula (Fig. 11, H), and this in turn be- 



