MAMMALIA. 



247 



open into the large blood-spaces already spoken of. From these spaces 

 there pass off oblique utero-placental veins, which pierce the serotina, and 

 form a system of large venous sinuses in the adjoining uterine wall (fig. 152, 

 F), and eventually fall into the general uterine venous system. At birth the 



FIG. 152. SECTION OF THE HUMAN UTERUS AND PLACENTA AT THE THIRTIETH 



WEEK OF PREGNANCY. (From Huxley after Ecker.) 



A. umbilical cord; B. chorion; C. foetal villi separated by processes of the 

 decidua serotina, D ; , F, G. walls of uterus. 



whole placenta, together with the fused decidua vera, and reflexa, with 

 which it is continuous, is shed ; and the blood-vessels thus ruptured are 

 closed by the contraction of the uterine wall. 



The fcetal membranes and the placenta of the Simiadae (Turner, No. 225) 

 are in most respects closely similar to those in Man ; but the placenta is, in 

 most cases, divided into two lobes, though in the Chimpanzee, Cynocephalus, 

 and the Apes of the New World, it appears to be single. 



The types of deciduate placenta so far described, are usually classified by 

 anatomists as discoidal placentae, although it must be borne in mind that 

 they differ very widely. In the Rodentia, Insectivora, and Cheiroptera there 

 is a (usually) dorsal placenta, which is co-extensive with the area of contact 

 between the allantois and the subzonal membrane, while the yolk-sack ad- 

 heres to a large part of the subzonal membrane. In Apes and Man the allan- 

 tois spreads over the whole inner surface of the subzonal membrane ; 

 the placenta is on the ventral side of the embryo, and occupies only a small 

 part of the surface of the allantois. The placenta of Apes and Man might be 



