MAMMALS. 



S4S 



(7) At the area of fixing, the uterine epithelium degene- 

 rates, the glands disappear, vascularity increases. 

 The outer part of the modified mucous membrane 

 (or decidua) becomes a spongy tissue, with spaces 

 filled with maternal blood. This maternal blood 

 bathes the trophoblast, which is intermediate between 

 it and the placental villi. 



The three modes of embryonic nutrition are as follows : — 



(a) At first the maternal blood bathes the lacunse in the 

 epiblastic outer wall — ^the trophoblast with its pre- 

 liminary path-finding villi. 



ip) An efficient yolk-sac placenta functions for a while, 

 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. 



The costomaky Classification of Placentation (which will probably be 



SOMEWHAT altered IN THE NEXT FEW YEARS) IS AS FOLLOWS ; — 



Meta-DiscoidaL — Villi at first scattered are \ tt„ „ j tvt 1 



restricted to a disc. f "'"^ ^""^ Monkeys. 

 The maternal mucous membrane forms a capsule around the 

 embryo (decidua reflexa, also seen in hedgehog). 



Caducous 



or 



Deciduate. 



(Vascular 



parts of 



maternal 



placenta 



come 



\ 



Discoidal.—y\\X\ on a circular cake-like f J^°*^^?*^^* , ™ - , 



^;^„ ■( Insectivora and Chiroptera. 



( Most Edentata. 



ICamivora. 

 Elephants and Hyrax. 

 Oryctero^us and Dasypus 

 among Edentata. 

 Dugong (in whole or in great 

 part non-deciduate). 

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

 or i 



Indeciduate. | 

 (Maternal 

 part of 



I Lemurs. 



plac™'ta°does | Diffuse.-^^Mt^^d. Villi. ] cetecST'^''" ^™'''" ^"™"™"- 



\ Manis among Edentata. 



not come away I 

 at birth). / 



.2 M 



