676 



MAMMALIA. 



Perameks the greater part — of the foetal 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. 



Caducous 



or 



Deciduate. 



(Vascular . 



parts of I 



maternal 



placenta 



come 



away 



at birth.) 



The customary Classification of Placentation. 



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

 restricted to a disc. / Monkeys. 



Around the embryo the maternal 

 mucous membrane forms a capsule 

 (decidua reflexa), also seen in hedge- 

 hog. 



Discoidal.- 



Zonary.- 



/Rodentia. 



I Insectivora (in the mole inde- 



,,..,. . , I ciduate and in part contra- 



-Villi onac.rcular J dedduate) and t hiroptera . 



cake - hkedlsc -\ Most Edentata. 



I Perameles (contra-decidu- 

 \ ate). 



fCarnivora. 

 Elephants and Hyrax. 

 Orycteropus and Dasypus 



among Edentata. 

 Dugong (wholly or in great 

 part non-deciduate). 



-Villi on a partial 

 or complete girdle 

 round the embryo. 



Non-Caducous iCotyledonary.- — Villi in patches. Ruminants. 



Indeciduate. 



(Maternal 



part of 



placenta does 



not come away 



at birth.) 



(Lemurs. 

 Most Ungulates, 

 Ruminants. 

 Cetacea. 

 Mam's among Edentata 



except 



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. 



The formation of the allantoic placenta in Perameles is in essentials 

 the same as in Eutherian Mammals, but in details there are some 

 striking differences. The most noteworthy of these is, perhaps, that the 

 cells of the uterine epithelium, instead of disappearing at an early stage, 

 as in Eutheiian Mammals, proliferate greatly, lose their cell outlines, 

 and by the increase of the nuclei form what is known as a syncytium. 

 Later this syncytial layer becomes highly vascular, and forms the 

 maternal portion of the placenta, whereas, as already seen, in Eutheria 



