248 THE ZONARY PLACENTA. 



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 decidu- 

 ate (Owen and Kolliker). Alf. Milne Edwards states that in Dasypus 

 novemcinctus the placenta is zonary, and both Kolliker 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 approaches the discoi- 

 dal type (Turner, No. 218). It occupies in Cholaepus Hoffmanni about four- 

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

 coid lobes. It is truly deciduate, and the maternal capillaries are replaced 

 by a system of sinuses (fig. 161). The amnion is close to the inner surface of 

 the chorion. A dome-shaped placenta is also found amongst the Edentata in 

 Myrmecophaga and Tamandua (Milne Edwards, No. 208). 



Zonary Placenta. Another form of deciduate placenta is known 

 as the zonary. This form of placenta occupies a broad zone of the chorion, 

 leaving the two poles free. It is found in the Carnivora, Hyrax, Elephas, and 

 Orycteropus. 



It is easy to understand how the zonary placenta may be derived 

 from the primitive arrangement of the membranes (vide p. 240) by the exten- 

 sion of a discoidal placental area to a sonary area, but it is possible that 

 some of the types of zonary placenta may have been evolved from the con- 

 centration of a diffused placenta (vide p. 261) to a zonary area. The 

 absence of the placenta at the extreme poles of the chorion is explained by 

 the fact of their not being covered by a reflection of the uterine mucous 

 membrane. In the later periods of pregnancy the placental area becomes, 

 however, in most forms much more restricted than the area of contact 

 between the uterus and chorion. 



In the Dog 1 , which may be taken as type, there is a large vascular yolk- 

 sack formed in the usual way, which does not however fuse with the chorion. 

 It extends at first quite to the end of the citron-shaped ovum, and persists 

 till birth. The allantois first grows out on the dorsal side of the embryo, 

 where it coalesces with the subzonal membrane, over a small discoidal area. 



Before the fusion of the allantois with the subzonal membrane, there 

 grow out from the whole surface of the external covering of the ovum, except 

 the poles, numerous non-vascular villi, which fit into uterine crypts. When 

 the allantois adheres to the subzonal membrane vascular processes grow 

 out from it into these villi. The vascular villi so formed are of course at 

 first confined to the disc-shaped area of adhesion between the allantois and 

 the subzonal membrane ; and there is thus formea a rudimentary discoidal 

 placenta, closely resembling that of the Rodentia. The view previously 

 stated, that the zonary placenta is derived from the discoidal one, receives 

 from this fact a strong support. 



The cavity of the allantois is large, and its inner part is in contact with 



1 Vide Bischoff, No. 175. 



