THE EARLY DEVELOPMENT OF THE MAMMAL 391 



place, but the basis of the phrase is clear. The enlargement of 

 the trophoblastic knob with its false amnionic cavity, pushes 

 the embryonic ectoderm cells far down within the cavity of the 

 blastodermic vesicle; this is especially marked in the guinea- 

 pig. Then as the endoderm cells spread out over the inner 

 surface of the embryonic ectoderm (Fig. 156) they may extend 



FIG. 156. Diagrams of the relations of the cavities and layers in the rat, 

 showing the "inversion" of the germ layers. After Selenka. Median sagittal 

 sections. Embryo and amnion, black; ectodermal knob or "trager" in light 

 tone; endoderm and mesoderm in darker tone. A. Early stage before the for- 

 mation of the false amnionic cavity. B. Late stage showing false and true 

 amnionic cavities and the interamnionic cavity, a, Amnion; ac, true amnionic 

 cavity; c, chorion; E, embryo (anterior end); ea, endodermal rudiment of 

 allantois; /, false amnionic cavity; i, interamnionic cavity; m, mesoderm; ma, 

 mesoderm of allantois; n, endoderm; o, trophoblast (ectoderm); p, anterior 

 intestinal portal; ra, rudiment of true amnionic cavity; rf, rudiment of false 

 amnionic cavity; s, marginal sinus; t, "trager" (ectoderm); y, yolk-sac; ye, yolk- 

 sac endoderm; x, amnionic folds. 



over the walls of both the true and the false amnionic cavities, 

 so that the embryonic ectoderm appears to lie within an endo- 

 dermal sac (Fig. 156, A). When the embryo itself begins to 

 be differentiated in the floor of the true amnionic cavity it is 

 thus already surrounded by this endodermal layer, and the 

 appearance of an inversion of the layers is produced. Con- 

 sideration of the entire series of conditions mentioned above 

 demonstrates the absence of a true inversion of the germ 

 layers. 



