386 OUTLINES OF CHORDATE DEVELOPMENT 



tiated into embryonic ectoderm and endoderm, but before 

 we continue the later history of these layers, we must consider 

 briefly two other matters, (a) the relation between the ecto- 

 derm of the embryonic shield and the trophoblast cells 



FIG. 151. Sections through four stages in the early development of the 

 Insectivor, Tupaija javanica. From Hubrecht. A. Blastodermic vesicle com- 

 pletely closed; endoderm still continuous with the embryonic ectoderm. B, C. 

 Embryonic ectoderm split and folding out upon the surface of the vesicle, 

 pushing away the trophoblast cells. D. Embryonic ectoderm forming a flat disc 

 on the surface of the blastodermic vesicle. E. Inner cell mass ("ectodermal 

 shield"); ec, embryonic ectoderm; en, endoderm; tr, trophoblast. 



(subzonal layer), and (6) the establishment of an important 

 cavity, the amnionic cavity, which first appears about this time. 

 There is a great deal of variation among the Mammals in the 

 details of these relations, and we shall attempt to give only a 

 very general statement of the more important conditions. 



