Chap, xni.] PLACENTA OF MAMMALS. . 515 



these membranes ; it will be remembered that, in 

 speaking of the extra-uterine development of the 

 embryo of the Sauropsida (the other division of 

 the Amniota, as the Mammalia and Sauropsida are 

 often collectively called) attention was directed to 

 the amnion, the allantois, and the yolk sac. As in 

 the other division, the embryo, notwithstanding the 

 smallness of the mammalian egg, is early distinguished 

 into an embryonic and a vitelline portion, but the 

 yolk sac * is always smaller than in Sauroids ; the 

 false amnion, or subzonal membrane, has, moreover, 

 an important part to play, which, from the nature of 

 things, was impossible in the oviparous groups, and 

 the yolk sac has, in some mammals, a certain re- 

 lation to the uterine walls. 



Dealing first with such a case (e.g. the rabbit) 

 we find the developing embryo soon becoming attached 

 to and embraced by the epithelium which lines the 

 uterus ', the epiblastic covering of the yolk sac 

 separates from the subjacent layers and unites with 

 the false amnion to form a layer beneath the zona 

 radiata (page 509) ; a little later the latter and the 

 subzonal membrane fuse together. Later on, 

 this subzonal membrane fuses with the vascular walls 

 of the yolk sac, to form a fresh investing membrane, 

 the false chorion. The true chorion is formed 

 by the fusion of part 'of the vascular wall of the 

 allantois with part of the subzonal membrane. 

 Into the substance of this chorion there extend blood- 

 vessels, and from its surface there grow out a number 

 of delicate processes which fit into corresponding 

 vascular " crypts " which are developed on the inner 

 wall of the uterus ; the whole of the apparatus thus 

 formed, partly by the mother and partly by the foetus, 

 is known as the placenta. 



* In mammals the yolk sac is generally known as the "um- 

 bilical vesicle." 



