576 ZOOLOGY SECT. 



some cases the allantois does not at first contain a cavity, and 

 in some (Primates) the severance between the amnion and the 

 chorion is not completed, and the allantois arises from the 

 outset in continuity with the latter. Sometimes, as in the Rabbit 

 (Fig. 1231), the union between the allantois (TA) and the chorion is 

 limited to a comparatively small part of the extent of the latter, 

 but in most instances the allantois spreads over the entire inner 

 surface of the chorion, and becomes united with it throughout 

 its entire extent (Fig. 1230). Villi, into which mesoderm with 

 blood-vessels penetrates, grow out from the surface of the chorion 

 and are received into depressions or crypts in the mucous membrane 

 of the uterus, which becomes profoundly modified. The villi 

 branch and enter into intimate union with the uterine mucous 

 membrane, so that a close connection becomes established between 

 the vascular system of the foetus and that of the parent. 



The term placenta is applied to the entire structure by means 

 of which this connection is brought about ; the parts derived from 

 the embryo are termed the foetal placenta, those developed from the 

 wall of the uterus the maternal placenta. In some Mammals the 

 union between the two is not very close, so that at birth no part 

 of the uterine mucous membrane is thrown off ; such a placenta is 

 said to be non-deciduate (semi-placenta). In other Mammals the 

 union is closer, and at birth a part of the hypertrophied mucous 

 membrane is thrown off in the form of a decidua ; such a placenta 

 is termed deciduate (placenta vera). In the Mole and the Bandicoot 

 not only is there no decidua thrown off, but the fcetal placenta 

 with the distal portion of the allantois does not pass out after the 

 foetus, but remains, and is broken up or absorbed in the uterus. 

 Such a condition has been termed contra-deciduate. 



In one of the simplest forms of placenta the discoidal found 

 in the Babbit and other Rodents (Fig. 1231), the yolk-sac extends 

 over the surface of the chorion and becomes fused with it, except 

 in a small area on the dorsal side of the embryo. In this small 

 area the allantois is applied to the chorion and coalesces 

 with it, and from the membrane so formed vascular villi grow out, 

 and are received into the uterine crypts. In most Mammals, 

 however, as already stated, the allantois becomes applied to the 

 chorion throughout its entire extent, and thus completely encloses 

 the embryo. Villi may be developed from all parts except the poles : 

 when this condition persists in the fully-formed placenta, the term 

 diffuse is applied. Sometimes the diffuse condition is temporary, 

 and the completed placenta has villi disposed in a broad band or 

 zone (zonary placenta). Sometimes the villi are grouped together 

 in patches or cotyledons (cotyledonary placenta). In Man and the 

 Apes the villi become secondarily restricted to a disc-shaped area 

 of the chorion situated on the ventral side of the embryo (meta- 

 discoidal placenta). 



