392 OUTLINES OF CHORDATE DEVELOPMENT 



G 



H 



FIG. 157. Diagrams of the formation of the amnion. After Keibel. A-F, 

 in Ruminants, pig, and Mammals, with entypy of the germ. G, H, in the Carniv- 

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

 endoderm; oblique shading, mesoderm. A. Morula. B. Early blastodermic 

 vesicle; endoderm limited to embryonic region. C. Formation of cavity in the 

 embryonic ectoderm (embryocyst). Extension of the endoderm. D. Embryo- 

 cyst opening out; embryonic disc becoming superficial. Endoderm completely 

 lining the vesicle. E. Embryonic disc or shield superficial in position (compare 

 H). F. Formation of amnionic folds, primitive streak, and exocoelom. G. Early 

 carnivor blastodermic vesicle. H. Embryonic shield superficial in position (com- 

 pare Fig. 154, B.) A later stage would be identical with F. a, Amnionic folds; 

 c. embryocyst; e, exocoelom; p, primitive streak; s, embryonic shield. 



