672 MAMMALIA. 



vascular processes or villi, which fit into corresponding 

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

 the mesoblastic wall of the allantois, plus the subzonal 

 membrane, the term " chorion " is sometimes applied, but 

 as the word has been used in many different senses, its 

 abandonment is almost imperative.] The complex union 

 of allantois with diplotrophoblast, Hubrecht calls the allan- 

 toidean trophoblast. (6) But in the hedgehog, rabbit, and 

 some other Eutherian types, as well as in certain Marsupials, 

 there is a mode of embryonic nutrition between that 

 attained by the epiblastic trophoblast and that effected by 

 the final placenta. The wall of the yolk-sac, hypoblastic 

 internally, mesoblastic externally, unites with the subzonal 

 membrane, and becomes the seat of villous processes, which 

 through the external epiblast are connected with the uterine 

 wall. Thus is formed what Hubrecht calls an omphaloidean 

 trophoblast or yolk-sac placenta. In connection 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 splitting of the mesoblast as it follows the 

 contour of the yolk-sac, forms a space — the extra-embryonic 

 body cavity, between the yolk-sac and the subzonal 

 membrane. When a yolk-sac placenta is developed, the 

 splitting of the mesoblast is retarded, so that the vascular 

 yolk-sac comes to lie close under the subzonal membrane. 

 This is especially well seen in Perameles (see Fig. 292), and 

 is of much importance in the formation of an efficient yolk- 

 sac placenta. 



(7) The embryo lay at first in a groove of the uterine 

 wall, moored by the preliminary blastocyst villi, which are 

 as it were pathfinders for those subsequently developed 

 from yolk-sac and allantoic regions. At the point of attach- 

 ment 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 

 exhibits unique blood spaces, forming what Hubrecht calls 

 the trophospongia ; below this there is the vascular and 

 vitally active remainder of the mucosa,- less modified than 



