CLASS/PICA TION OF PL A CENT A TION. 67 5 



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 diplotrophoblast. This yolk- 

 sac placenta is well seen in Insectivora, Chiroptera, Rodentia, 

 the horse, etc. , and seems to be to some extent developed in all 

 Mammals (except Monotremes) as yet examined. 



(c) The final placenta is allantoidean. 



In the above description the yolk-sac placenta has been emphasised 

 on account of its comparative importance, but it must be clearly under- 



FlG. 296. — View of embryo, with its fcetal membranes. 

 — After Kennel. 



am., Amnion proper ; d., dwindled yolk-sac ; al., allantois ; al.' , 

 subzonal membrane \z., villi. Outside the subzonal mem- 

 brane there is the delicate ectodermic trophoblast iz.'). 



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 fcetal 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 the foetal villi are simply withdrawn from the maternal crypts, and the 

 placenta is indeciduate. In Perameles, and to a less extent in the mole 

 \Talpa), not only is there no loss of maternal tissue, but part — in 



