206 THE ZONARY PLACENTA. 



Zonary Placenta. Another form of decidiuite placenta is known as 

 the zonaiy. 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 zonaiy placenta may be derived from 

 the primitive arrangement of the membranes (vide p. 199) by the exten- 

 sion of a discoidal placenta! area to a zonary area, but it is possible 

 that some of the types of zonary placenta may have been evolved from 

 the concentration of a diffused placenta (ride p. 217) 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 subzoual membrane vascular processes 

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



o 



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

 and the subzonal membrane ; and there is thus formed, a rudimentary dis- 

 coldid placenta, closely resembling that of the Rodentia. The view pre- 

 viously 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 the amuion. The area of adhesion between the outer part of the allantois 

 and subzonal membrane gradually spreads over the whole interior of the 

 subzonal membrane, and vascular villi are formed over the whole area of 

 adhesion except at the two extreme poles of the egg. The last part to be 

 covered is the ventral side where the yolk-sack adjoins the subzonal 

 membrane. 



During the extension of the allantois its cavity persists, and its inner 

 part covers not only the amnion, but also the yolk-sack. It adheres to 

 the amnion and supplies it with blood-vessels (Bischoff). 



With the full growth of the allantois there is formed a broad placental 

 zone, with numerous branched villi, fitting into corresponding pits which 

 become developed in the uterine walls. The maternal and fcetal struc- 

 tures become closely interlocked and highly vascular ; and at birth a large 

 part of the maternal part is carried away with the placenta ; some of it 

 however still remains attached to the muscular wall of the uterus. The 

 villi of the chorion do not fit into uterine glands. The zone of the pla- 

 centa diminishes greatly in proportion to the chorion as the latter elongates. 

 and at the full time the breadth of the zone is not more than about one- 

 fifth of the whole length of the chorion. 



1 Vide Bischoff, No. 175. 



