EARLY MAMMALIAN DEVELOPMENT. 



as wholly comparable to invagination or epiboly or involution in the lower 

 forms. The results are arrived at by abbreviated or caenogenetic steps in 

 which the characteristics of the lower forms are profoundly modified. 



Taking the bat again as an example, some of the cells of the inner cell 

 mass bordering the cavity of the blastocyst, or yolk cavity, become differen- 

 tiated, proliferate and migrate around on the inner surface of the trophoderm, 

 forming there a complete layer and lining for the cavity. This new layer is 

 the primary entoderm (Fig. 59, a; compare with Fig. 55, d). Immediately 

 following the formation of the entoderm many of the cells of the inner cell 

 mass become vacuolated and disappear, resulting in the formation of a cavity 

 between the overlying trophoderm and the cells contiguous to the entoderm. 

 This new cavity is the amniotic cavity (Fig. 59, b and c). The two cavities are 



fern. 



1 fefeS^ 



FIG. 60. Three stages in the formation of the germ layers in the lemur, Tarsius spectrum. 

 Hubrecht, from Quain's Anatomy, fcm., Inner cell mass; emb. ect., embryonic ectoderm; 

 ent., entoderm. 



separated therefore by a double-layered plate, the embryonic disk, the layer 

 bordering the amniotic cavity comprising the embryonic ectoderm (Fig. 59, c). 

 In the lemur, Tarsius spectrum, the differentiating entodermal cells, instead 

 of spreading over the inner surface of the trophoderm, form a small sac 

 within the larger cavity (Fig. 60, a, b, c). In Tarsius the inner cell mass 

 seems to invaginate from its outer side and the definite layer of cells thus 

 resulting becomes the embryonic ectoderm (Fig. 60, c). Subsequently the 

 inverted ectoderm becomes straightened out so that the embryonic disk is 

 flat. In either case the embryonic disk is destined to give rise to the' 

 embryonic body. 



In the white rat the progress of development following the formation of 

 the blastocyst is marked by a curious inversion of the inner cell mass, giving 

 rise to a condition known as inversion or entypy of the germ layers. This 



