7io 



MAMMALIA. 



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, are 



re>tricted to a disc. 



Homo and 

 Monkeys. 



j^i 



Around the embryo the maternal 

 mucous membrane forms a capsule 

 (decidua reflexa), also seen in hedge- 

 hog. 



(Rodentia. 

 Insectivora(inthemole inde- 

 ciduate and in part contra- 

 utt,. v mi uu a. uuuuuti deciduate) and Chiroptera. 

 cake-like disc. J Most Edentata. 



[ Perameles (contra - decidu- 

 \ ate). 





"F s 



Carnivora. 



Elephants and Hyrax. 



A T\ 



Orycteropus and Das 



sypus 



us / 



TAT I \ among Edentata, 

 round the embryo. DugO ng (wholly or in great 



\ part non-deciduate). 

 Co/yledonary. \'illi in patches. Ruminants. 



Non-Caducous 



or 



Indeciduate 

 (Maternal 



part of 

 placenta does \Diffuse. Scattered Yilli. { 



not come away I 



nf hirth \ V 



( Lemurs. 

 Most Ungulates, 

 Ruminants. 



at birth.) 



except 



Cetacea. 



Mam's 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. 



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 Eutherian 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 



