CLASSIFICATION OF PLACENTATION. 



655 



(//) An efficient yolk sac placenta functions for a time, 

 but decreases and shrivels as the final allantoidean 

 placenta develops. The maternal blood in the spaces 

 of the outer layer of the mucous layer of the uterus 

 bathes the trophoblast. Thus it comes into indirect 

 connection with the vascular villi from the region 

 where the yolk-sac wall unites with the diplotropho- 

 blast. This yolk sac placenta is found in Insectivora, 

 Chiroptera, and Rodents. 



{c) The final placenta is allantoidean, it replaces the 

 yolk-sac placenta, if there be one. In Insectivora 

 Chiroptera, and Rodentia, and probably in other 

 cases, the trophoblast is always intermediate between 

 the maternal blood and the villi, and is the only 

 intervening tissue. 



Caducous 



or 



Deciduate. 



(Vascular 



parts of 



maternal 



placenta 



come 



away 



at birth). 



The custom.^ky Classification of Placentation 

 is as follows : — 



Meta-Discoidal. — Villi at first scattered 2S(^\Honio and 

 restricted to a disc. j Monkeys. 



The maternal mucous membrane forms a 

 capsule around embryo (decidua retlexa , 

 also seen in hedgehog). 



Disi'oidal.- 



-Villi on 



cake-like disc 



circular r?°'3™'i=^- ^ „, . , 



-J Insectivora and Chiroptera. 



Zonarv. 



-Villi on a partial 

 or complete girdle 

 round the embryo. 



I^Most Edentata. 



Carnivora. 



Elephants and Hyrax. 



Oryctei-opiis and Dasypus 



among Edentata. 

 Dugong (in whole or in great 



part non-deciduate). 



Non-Caducous ' 



or 



Indeciduate. 



(Maternal 



part of 



placenta does 



not come away 



at birth). 



Cotyledonmy,- — Villi in patches. Ruminants. 



{Lemurs. 

 Most Ungulates except 

 Ruminants. 

 Cetacea. 

 Manis among Edentata. 



[Table. 



