824 PHYLUM CHORDATA : CLASS MAMMALIA 



endoderm, but it is covered externally by a layer of 

 mesoderm, which it bears with it as it grows. In all 

 placental Mammals, the allantois, which becomes richly 

 vascular, unites with the subzonal membrane, and there- 

 fore with the external ectoderm or trophoblast as well, 

 to form the foetal part of the placenta, with outgrowing 

 vascular processes or villi, which fit into corresponding 

 depressions or crypts on the wall of the uterus. To the 

 mesodermal wall of the allantois, plus the subzonal mem- 

 brane, the term " chorion " is sometimes applied ; but as 

 the word has been used in many different senses, its 

 abandonment is almost imperative. (6) But in the hedge- 

 hog, rabbit, and some other Eutherian types, as well as in 

 certain Marsupials, there is a mode of embryonic nutrition 

 between that attained by the trophoblast and that affected 

 by the final placenta. The wall of the yolk-sac, endodermal 

 internally, mesodermal externally, unites with the subzonal 

 membrane, and becomes the seat of villous processes, which 

 through the external ectoderm are connected with the 

 uterine wall. Thus is formed a yolk-sac placenta. In con- 

 nection with this yolk-sac placenta it will be recollected 

 that the yolk-sac, here as in the Bird, is a vascular structure 

 well fitted for a placental function. In the Bird and in 

 most Mammals, however, the splittin<^ of the mesoderm as 

 it follows the contour of the yolk-sac forms a space — the 

 extra-embryonic body cavity — between the yolk-sac and 

 the subzonni membrane. When a yolk-sac placenta is de- 

 veloped,the splitting of the mesoderm is retarded, so that 

 the vascular yolkcc comes to lie close under the subzonal 

 membrane. This is especially well seen in Perameles (see 

 Fig. 493), and i^ of much importance in the formation of 

 an efficient yolk-sac placenta. 



(7) The embryo lies at first in a groove of the uterine wall, 

 moored by the preliminary blastocyst viUi, which are as it 

 were pathfinders for those subsequently developed from 

 yolk-sac and allantoic regions. At the point of attachment 

 the mucous lining of the uterus ceases to be glandular, and 

 becomes much more vascular. As the embryo becomes 

 fixed, the blastocyst almost eating its way in, the outer 

 epithelium degenerates and disappears ; below this the 

 next layer of the mucous membrane becomes spongy and 



