CAUSE OF HEREDITY 



269 



into two cells. Each of the two thus produced divides 

 again, and this process continues until the whole body of 

 the multicellular animal or plant is built up. Among the 

 cells of the earlier generation there is a great similarity 



^-J . I -> 



FIG. 40. How cells multiply and begin building _.. _ _. . ., 



5, section through blastula showing hollow sphere; 6, gastrula showing outer 



4, morula: 



layei of cells (epiblast) and inner layer (hypoblast) ; the 6 is at the mouth of 

 tht cavity (enteron) of the gastrula. HAEC 



in appearance, in fact, until a great number of cells have 

 been produced, it is impossible to see any difference be- 

 tween them. As in the higher animal, the cells that are 

 eventually going to be thrown off to form new individuals, 

 the sexual cells, are differentiated at a very early period, 



