VARIETIES OF PLACEKTiK. 431 



with them the uterine mucous membrane comes away with the 

 placenta. We thus get two forms of placenta, the non-deciduate 

 and the deciduate. The non-deciduate placenta comes away 

 from the uterine mucous membrane separately, whilst in the 

 deciduate form both are so closely interlocked that they come 

 away as the " afterbirth " together. 



Varieties of Placenta?. 



We can distinguish five kijids of placentse — viz., the dis- 

 coidal, the meta-discoidal, the zonary, the ootyledonary, and 

 the diffuse placenta. The discoidal is found in the Eodents, 

 Bats, and Insectivora ; tlie meta-discoidal in Man and Apes ; 

 the zonary in Carnivora, Elephants, and the Hyrax ; the cotyle- 

 donary in the Euminants ; the diffuse in the Horse, Pig, &c. 



1. The discoidal lilacenta. — The amnion is small and sur- 

 rounds the embryo. The yolk-sac is attached to the embryo by 

 a very long stalk, and is closely a])plied to the greater part 

 of the sub-zonal membrane, forming the fal&e eliorion. The 

 allantois forms a moderate-sized sac, which below is applied to 

 the sub-zonal membrane, and there forms the true chorion or 

 placenta. From this placental area pass vascular villi into 

 the uterine pits. This discoidal placenta is deciduate. A great 

 space exists between tlie embryo and the false chorion, whicli 

 is filled with fluid. 



2. The meta-dincxiidal placenta (fig. 205). — Here a complete 

 chorion is formed, tlie allantois entirely surrounding the inner 

 wall of the sub-zonal membrane. The embryo is encircled by 

 the amnion, which swells out almost to the placenta, only being 

 separate from it by a gelatinous layer. The yolk-sac is 

 very small. As the blastodermic vesicle grows it becomes 

 related to three parts of the uterus. These three parts are 

 thrown out at birth and are called decidua. One part of the 

 Uterus is reilected over the vesicle = the decidua refexa {/) , 

 that part of the uterus around which the reflexa is attached is 



