XI.J DISCOIDAL PLACENTA. 353 



centa presents a great variety of forms, and we propose 

 first to treat the most important of these in succession, 

 and then to give a general exposition of their mutual 

 affinities. 



The discoidal placenta is found in the Rodentia, 

 Insectivora, and Cheiroptera. The Rabbit may be 

 taken as an example of this type of placenta. 



The Babbit. In the pregnant female Eabbit several ova are 

 generally found in each horn of the uterus. The general condi- 

 tion of the fcBtal-membranes at the time of their full development 

 is shewn in Fig. 116. 



The embryo is surrounded by the amnion, which is compara- 

 tively small. The yolk-sao (ds) is large and attached to the 

 embryo by a long stalk. It has the form of a flattened sac 

 closely applied to about two-thirds of the surface of the subzonal 

 membrane. The outer wall of this sac, adjoining the subzonal 

 membrane, is formed of hypoblast only ; but the inner wall is 

 covered by the mesoblast of the area vasculosa, as indicated by 

 the thick black line (fd). The vascular area is bordered by 

 the sinus terminalis (st). In an earlier stage of development the 

 yolk-sao had not the compressed form represented in the figure. 

 It is, however, remarkable that the vascular area never extends 

 over the whole yolk-sac ; but the inner vascular wall of the yolk- 

 sac fuses with the outer wall, and with the subzonal membrane, 

 and so forms a false chorion, which receives its blood supply 

 from the yolk-sac. This part of the chorion does not develop 

 vascular villi. 



The aUantois (al) is a simple vascular sac with a large cavity. 

 Part of its wall is apphed to the subzonal membrane, and gives rise 

 to the true chorion from which there project numerous vascular 

 vilU. These fit into corresponding uterine crypts. It seems pro- 

 bable, from Bischoflfs and Kblliker's observations, that the sub- 

 zonal membrane in the area of the placenta becomes attached, 

 by means of viUi, to the uterine wall even before its fusion with 

 the allantois. In the later periods of gestation the intermingling 

 of the maternal and foetal parts of the placenta becomes very 



F. & B. 23 



