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 of 
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 foetal tissues. Where the connection is very 
intimate, parts of the maternal tissue come away at birth, and the 
placenta is said to bedeciduate. Where there is a less close interlocking, 
the foetal villi are simply withdrawn from the maternal crypts, and the 
placenta is indeciduate. In Perameles, and toa less extent in the mole 
(Tal~a), not only is there no loss of maternal tissue, but part—in 
Perameles the greater part—of the foetal portion of the placenta is 
absorbed 77% sztu 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 
Meta-Discotdal.—Villi, at first scattered, are Homo and 
restricted to a disc. f Monkeys. 
Around the embryo the maternal 
mucous membrane forms a capsule 
Caducous ; (decidua reflexa), also seen in hedge- 
or hog. 
Deciduate. 
(Vascular Rodentia. 
parts of Insectivora (in the mole inde- 
maternal ( Discotdal.—Villi on a circular} Ciduate and in part contra- 
placenta cake-lilce disc: deciduate)and Chiroptera. 
come Most Edentata, 
away [nets (contra - decidu- 
at birth.) ate). 
Carnivora. . 
Elephants and Hyrax. 
Oryeteropus and Dasypus 
among Edentata. 
Dugong (wholly or in great 
part non-deciduate). 
Non-Caducous / Cotyledonary.—Villi in patches. Ruminants. 
Zonary.—Villi on a partial 
or complete girdle 
round the embryo. 
or 
Indeciduate. 
(Maternal Lemurs. 
part of Most Ungulates, except 
placenta does | Dzfuse.—Scattered Villi.; Ruminants. 
not come away Cetacea. 
at birth.) Manis among Edentata. 
