44 THE BIOLOGY OF TWINS 



up and the process of rounding up is unquestionably 

 a sort of invagination of the middle or apical portions, so 

 that the free edges unite above and the whole mass 

 becomes essentially a hollow ball. In Fig. ii the ball 

 appears to be only partially hollow, but the ectodermic 

 mass is morphologically a vesicle with a central cavity, 

 which is the primitive amniotic cavity. This invagi- 

 nation of the ectoderm involves a very fundamental 

 reversal of the primary axis of the embryonic 

 materials, for the head end' of the ectodermic mass is 

 now directed away from the original animal pole of 

 the egg. 



The ectoderm is the active agent in this process of 

 germ-layer inversion, and the endoderm plays the 

 merely passive role of maintaining its contact with 

 the ectoderm. The result is that it comes almost 

 completely to surround the ectodermic vesicle. As a 

 sequel to the inversion process the ectoderm becomes 

 totally separated from contact with the trophoblast, 

 and a cavity arises between the latter and the embryonic 

 tissues, which is the beginning of the extra-embryonc 

 cavity {ex c) . 



That part of the trophoblast which has adhered 

 to the uterine mucosa has at this period entered upon 

 a process of rapid cell proliferation preparatory to 

 invading the maternal tissues. At this time only short 

 protrusions have been formed that serve as mechanical 

 aids to adhesion. This specialized region of the tropho- 

 blast is destined to form the primitive placenta or 



^ It may be noted here that in later stages (Figs. 15, 16, and 17) the 

 heads of all embryos are directed away from the original apical end 

 or animal pole of the egg, which is the attached end. 



