THE EARLY DEVELOPMENT OF THE MAMMAL 393 



An interesting condition, transitional between the two 

 general classes described above, is found in the mole. Here 

 spaces form between the ectoderm and the continuous tropho- 

 blast, which flow together as in the bat, forming a definite 

 amnionic cavity. This cavity is then obliterated by the re- 

 fusion of the ectoderm and trophoblast, resulting in a con- 

 dition similar to that of the shrew and rabbit. Then the 

 superficial cells of the trophoblast disappear, leaving the ecto- 

 derm on the surface of the blastodermic vesicle. 



The formation of the amnionic cavity has not been mentioned 

 in forms where the embryonic disc becomes superficial in posi- 

 tion. In these instances the amnionic cavity is formed in a 

 manner entirely different from that described above. Here 

 a system of amnionic folds, much like those of the chick, appears 

 just outside the embryonic region proper. These folds grow 

 up over the embryo, establishing an amnionic cavity in which 

 the embryo is enclosed (Figs. 157, 176). The history of the 

 amnion will be taken up later, in connection with the other 

 embryonic membranes. 



We may consider now the processes leading to the formation 

 of the definitive embryo. To give a comparative account of 

 these processes as they occur in the whole group of Mammals 

 would carry us far into details, as there is wide variation even 

 within this single class, and we shall, therefore, describe these 

 phenomena as they are found in such a form as the rabbit or 

 dog, forms whose early history is comparatively well known. 



Of the entire blastodermic vesicle, with its complicated 

 associated structures, the only truly embryonic portion is 

 found in a restricted portion of the embryonic ectoderm and 

 endoderm known as the embryonic shield (Figs. 151-155, 157, 

 158, A). (We should note that in the rabbit the endoderm 

 does not come to line the entire blastodermic vesicle until a 

 relatively late stage, so that throughout early development 

 only the upper portion of the vesicle is two-layered.) In the 

 rabbit the embryonic shield first becomes visible about the 

 fifth day, when the blastodermic vesicle is still spherical and 

 only about 1.5 mm. in diameter. By the seventh day the 



