PLACENTATION 



827 



From the comparative standpoint the most important variations in 

 regard to the placenta are — first, the method of distribution of the vilH 

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

 between maternal and foetal 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 the foetal villi are simply withdrawn from the maternal cr^'pts, and the 

 placenta is indeciduate. In Peranieles, 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 



Caducous 



or 



Deciduate. 



(Vascular 



parts of 



maternal 



placenta 



come 



away 



at birth.) 



Meta-Discoidal. — Villi, at first scattered, axe\Homo and 



restricted to a disc. j Monkeys. 



Around the embryo the maternal 

 mucous membrane forms a capsule 

 (decidua reflexa), also seen in hedge- 

 hog. 



fRodentia. 

 Insectivora (in the mole inde- 

 Discoidal—\\\Yx on a cir- ciduate and in part contra- 

 deciduate) and ChLroptera. 

 Most Edentata. 

 Perameles (contra - decidu- 

 ate). 



-Villi on a cir- 

 cular cake-like \ 

 disc. 



Zonary. 



—Villi on a partial 

 or complete girdle <[ 

 round the embryo. 



' Carnivora. 



Elephants and Hyrax. 

 Orycteropus and Dasypus 



among Edentata. 

 Dugong (wholly or in great 

 part non-deciduate). 

 Non-Caducous ( Cotyledonary . — Villi in patches. Ruminants, 

 or 

 Indeciduate. 

 (Maternal 

 part of 

 placenta does 

 not come away 

 at birth.) 



,' Lemurs. 



j Most Ungulates, except 

 Diffuse. — Scattered ViUi.-; Ruminants. 



I Cetacea. 

 \Manis among Edentata. 



There is some uncertainty as to the primitive form of the placenta, 

 but the fact that it is discoidal in Perameles seems to confirm Balfour's 

 view that this form must be placed lowest. 



