THE EARLY DEVELOPMENT OF THE MAMMAL 389 



indistinguishably with the cells of the ectoderm itself ; in either 

 case the trophoblast cells here disappear as such (Figs. 154, 

 157, G, H). 



In the second condition mentioned, where the trophoblast 

 remains continuous above the embryonic layers (entypy), we 

 find a still greater variety in the details of the relations, which 

 may become very complicated. This general type of develop- 

 ment is found in many Primates (probably in man), many 

 Rodents including the mouse, rat, and guinea-pig, in the 

 Chiroptera (Fig. 155), and in some Insectivora including the 

 hedgehog (Erinaceus) Galeopithecus and Gymnura. In all these 

 forms a space appears in the ectoderm, known as the amnionic 

 cavity. This space may result from a definite splitting apart of 

 two cell masses (e.g., hedgehog), or it may result from the 

 gradual confluence of irregular spaces (e.g., bat, Fig. 155). It 

 is important to bear in mind that in these instances of entypy, 

 the true embryo develops only from the cells (ectoderm and 

 endoderm) lying in the floor of the amnionic cavity. 



Very often the trophoblast in this region becomes thickened, 

 forming a trophoblastic knob ("trdger"), which enlarges and 

 pushes the embryonic shield down into the cavity of the blasto- 

 dermic vesicle (rat and mouse, for example). An additional 

 cavity, the false amnionic cavity, may develop within this 

 trophoblastic knob. This should not be confused with the 

 true amnionic cavity, which here forms a completely closed 

 vesicle, with ectodermal wall, entirely separate from the tro- 

 phoblastic knob, and often of considerable size, even in these 

 early stages (Fig. 156). A rather more special condition is 

 found in the guinea-pig, where the small amnionic vesicle 

 separates widely from the trophoblast, leaving a large space 

 known as the interamnionic cavity between the true and the 

 false amnionic cavities. The false amnionic cavity of the tro- 

 phoblastic knob becomes very large (Fig. 156, B), while during 

 these early stages the true amnionic cavity remains very small. 



It is among these forms (e.g., rat, guinea-pig, etc.) that the 

 phenomenon of the so-called "inversion of the germ layers" 

 was described. It is now clear that no genuine inversion takes 



