THE EARLY DEVELOPMENT OF THE MAMMAL 385 



subzonal cells, with the inner cell mass suspended from its 

 inner surface like a hanging drop (Figs. 148, C; 149). The 

 first step in the differentiation of this structure consists in the 

 rearrangement of the cells of the inner mass so that those bor- 

 dering the cavity of the vesicle are formed into a definite, con- 

 tinuous layer, known as the embryonic endoderm (Figs. 152, 155). 

 The relative time at which the endoderm becomes distinct may 

 vary greatly. The endoderm cells multiply rapidly, and typ- 

 ically they spread distally over the entire inner surface of the 

 subzonal layer, converting the blastodermic vesicle into a two- 



FIG. 150. Diagrams of the formation of the amnion in the Insectivors. After 

 Keibel. Black, embryonic ectoderm; heavy stipples, trophoblast; light stipples, 

 endoderm; oblique ruling, mesoderm. A. Before the appearance of the amnionic 

 cavity. Inner cell mass differentiated into embryonic ectoderm and mesoderm; 

 endoderm extending completely around the wall of the vesicle. B. The amnionic 

 cavity (a) appearing in the ectoderm. C. Enlargement of the amnionic cavity. 

 Mesoderm expanded and split into somatic and splanchnic layers, separated by 

 the ccelom. s, Primitive streak. 



layered structure, the gastrula (Figs. 150, 153, 155). In the 

 Primates, however, the formation of the endoderm does not 

 keep pace with the enlargement of the blastodermic vesicle, 

 so that the endoderm forms a smaller second vesicle suspended 

 below the inner cell mass (Fig. 161). 



After the separation of the endoderm, the remainder of the 

 inner cell mass is known as the embryonic ectoderm. In most 

 instances this remains quite distinct from the original subzonal 

 layer, which, in view of its future function of attaching the 

 vesicle to the uterine mucosa, may now be termed the tropho- 

 blast (Hubrecht). The inner cell mass is now clearly differen- 



