THE EARLY DEVELOPMENT OF THE MAMMAL 387 



We may make a preliminary distinction between those in- 

 stances where the trophoblast is interrupted in a circumscribed 

 area, just above the embryonic ectoderm, the embryonic disc 

 then moving up and occupying this space, and thus acquiring a 

 superficial position; and others where the trophoblast remains 



FIG. 152. Three stages in the formation of the embryonic shield in the deer. 

 After Keibel. A. Spaces appearing in the inner cell mass. B. Fully formed 

 cavity (embryocyst) in the inner cell mass, which is still covered with a thin 

 trophoblast. C. Embryocyst opened out upon the surface, c, Ectoderm ; i, inner 

 cell mass; n, endoderm; s, embryonic shield; t, trophoblast; y, embryocyst. 



continuous above the embryonic ectoderm, forming a complete 

 enveloping layer around the endoderm and embryonic shield. 

 The latter condition is known as the entypy of the germ. 



Among the former instances we find again quite a variety of 

 methods by which the embryonic layers acquire their super- 

 ficial position. (1) In the Insectivor, Tupaija, the inner cell 

 mass forms a cup-shaped structure which opens out upon the 

 surface of the vesicle, pushing the trophoblast away (Fig. 151). 

 (2) More commonly, as in most Ungulates, in Tarsius, and the 



