MAMMALIA. 



205 



fused decidua vera, and i eflexa, with which it is contiiuioiis, is shed ; and 

 the bh)od vessels thus 

 ruptvired are closed 

 by the contraction of 

 the uterine wall. 



The foetal mem- 

 branes and the pla- 

 centa of the Simiadee 

 (Turner, No. 225) are 

 in most respects close- 

 ly similar to those in 

 Man ; but the pla- 

 centa is, in most cases, 

 divided into two lobes, 

 though in the Chim- 

 panzee, Cynocephalus, 

 and the Apes of the 

 New World, it appears 

 to be single. 



The ty}>es of de- 

 ciduate placenta so 

 far described, are usu- 

 ally classified by ana- 

 tomists as discoidal 

 placentae, although it 

 must be borne in 

 mind that they differ 

 very widely. In the 

 Rodentia, Inse.cti- 

 vora, and Cheiroptera 

 there is a (usually) dorsal placenta, which is co-extensive with the area of 

 contact between the allantois and the subzonal membrane, while the yolk- 

 sdck adheres to a large part of the subzonal membrane. In Apes and 

 Man the allantois spreads over the whole inner surface of the subzonal 

 membrane ; the placenta is on the ventral side of the embryo, and occupies 

 only a small part of the surface of the alhuitois. The placenta of Apes 

 and Man might be called metadiscoidal, in order to distinguish it from the 

 primitive discoidal placenta of the Rodentia and Insectivora. 



In the Armadilloes (Dasypus) the placenta is truly discoidal and deci- 

 duate (Owen and Kblliker). Alf. Milne Edwards states that in Dasypus 

 novemcinctus the placenta is zonary, and both Kiilliker and he found four 

 embryos in the uterus, each with its own amnion, but the placenta of all 

 four united together ; and all four enclosed in a common chorion, A 

 reflexa does not appear to be present. In the Sloths the placenta ap- 

 proaches the discoidal type (Turner, No. 218). It occupies in Chselopus 

 Hoffmanni about four-fifths of the surface of the chorion, and is composed 

 of about thirty-four discoid lobes. It is truly deciduate, and the matei-nal 

 capillaries are re])laced by a system of sinuses (fig. 161)., The amnion is 

 close to the inner surface of the chorion. A dome-shaped placenta is also 

 fovmd amongst the Edentata in Myrmecophaga and Tamandua (Milne 

 Edwards, No. 208). 



Fig. 152. Sf:ction of the human utekus and placenta 

 AT THE THIRTIETH WEEK OF PREGNANCY. (From Huxley, after 

 Ecker.) 



A. umbilical cord; B. chorion; C. foetal villi separated 

 by processes of the decidua serotina, D ; E, F, G. walls 

 of uterus. 



