CHAPTER XI. 



EMBRYONIC MEMBRANES AND YOLK-SAC. 



In the Mammalia the early stages in the develop- 

 ment of the embryonic membranes are nearly the same 

 as in Aves ; but during the later stages the allantois 

 enters into peculiar relations with the uterine walls, 

 and the two, together with the interposed portion of 

 the subzonal membrane or false amnion (the nature of 

 which will be presently described), give rise to a very 

 characteristic Mammalian organ — the placenta — into 

 the structure of which it will be necessary to enter 

 at some length. The embryonic membranes vary so 

 considerably in the different forms that it will be ad- 

 vantageous to commence with a description of their 

 development in an ideal case. 



We may commence with a blastodermic vesicle closely 

 invested by the delicate remnant of the zona radiata at 

 the stage in which the medullary groove is already 

 established. Around the embryonic area a layer of 

 mesoblast would have extended for a certain distance ; 

 so as to give rise to an area vasculosa, in which how- 

 ever the blood-vessels would not have become definitely 



