734 



MAMMALIA. 



stood that the allantoic placenta is often the only one well developed, 

 and is always of supreme importance in reference to the nutrition oi 

 the embryo. 



From the comparative standpoint the most important variations in 

 regard to the placenta are first, the method of distribution of the villi 

 on the surface of the allantois ; and second, the extent of the connection 

 between maternal and fcetal tissues. Where the connection is very 

 intimate, parts of the maternal tissue come away at birth, and the 

 placenta is said to be deciduate. Where there is a less close interlocking, 

 the fcetal villi are simply withdrawn from the maternal crypts, and the 

 placenta is indeciduate. In Perameles, and to a less extent in the mole 

 (Talpa)) not only is there no loss of maternal tissue, but part in 

 Perameles the greater part of the fcetal portion of the placenta is 

 absorbed in situ by maternal leucocytes, a condition described by 

 Hubrecht as contra-deciduate. The distinction between the deciduate 

 and indeciduate forms is not perfectly sharp, and Hubrecht prefers the 

 older terms, Caducous and Non-Caducous. 



THE CUSTOMARY CLASSIFICATION OF PLACENTATION 



\Mtta-Discoidal. Villi, at first scattered, 2xo\Homo and 

 restricted to a disc. / Monkeys. 



Caducous 



or 



Deciduate. 

 (Vascular 

 parts of 

 maternal 

 placenta 



come 



away 

 at birjh.) 



Around the embryo the maternal 

 mucous membrane forms a capsule 

 (decidua reflexa), also seen in hedge- 

 hog. 



fRodentia. 



I Insectivora(inthemoleinde- 



DiscoidaL Villi on a circular 5 iduate and "I Pf . fc contra - 

 cake-like disc. \ deciduate) and Chiroptenu 

 Most Edentata. 

 Peraweles (contra - decidu- 

 ate). 



Non-Caducous 



or 



Indeciduate. 



(Maternal 



part of 



placenta does 



not come away 



at birth.) 



Carnivora. 



Elephants and Hyrax. 

 Orycteropus and Dasypus 



among Edentata. 

 Dugong (wholly or in great 



part non-deciduate). 

 Cotyledonary. Villi in patches. Ruminants. 



Zonary. Villi on a partial 



or complete girdle 



< round the embryo. 



rLemurs. 



Most Ungulates, except 

 Diffuse. Scattered Villi. -! Ruminants. 



Cetacea. 

 \Manis among Edentata. 



