162 GENERAL BIOLOGY 



mentioned, the blastopore. In many eggs the invag- 

 inating process stops before the blastocoele is oblit- 

 erated, but the same relations of parts obtain. 

 In eggs in which yolk is very abundant, such as 

 those of the frog, the yolk-laden part of the zygote 

 cleaves into much larger cells (macromeres) than 

 the opposite region, so large, in fact, that these 

 cells bulk greater than the segmentation cavity of the 

 blastula, and it is physically impossible for these 

 cells to invaginate into such a cavity. The same 

 result is reached, however, by a growth of the smaller 

 upper-layer cells over the macromeres, so that the 

 latter disappear within the former. The final result 

 is essentially the same as that previously described, 

 viz. a two-layered gastrula with a central cavity 

 (archenteron), communicating with the exterior by a 

 blastopore. 



Further Differentiation. In the higher verte- 

 brates the process of gastrulation is greatly modified 

 and obscured by the nature of the cleavage, but 

 essentially the same change takes place in all animals, 

 the conversion of a single-layered blastula into a 

 double-layered gastrula. The outer layer of the 

 gastrula is called ectoderm, the inner layer, endoderm. 

 There quickly develops between the two, as an out- 

 growth, mainly from the endoderm, a third layer or 

 mass of cells known as the mesoderm or middle 

 layer. From these three germ-layers there are sub- 

 sequently differentiated all the tissues and organs 

 of the animal body. From the ectoderm develops 



