CONNECTION BETWEEN EMBRYO AND MOTHER. 731 



blast, and the mesoblastic subzonal membrane, are included 

 in Hubrecht's term diplotrophoblast. (5) From the hind- 

 wall of the gut there grows out a hypoblastic sac, the 

 allantois, insinuating itself and spreading out in the space 

 between the two layers of mesoblast. As an outgrowth of 

 the gut homologous with the bladder of the frog, the 

 allantois is lined by hypoblast or endoderm, but it is 

 covered externally by a layer of mesoblast, which it bears 

 with it as it grows. In all placental Mammals, the allantois, 

 which becomes richly vascular, unites with the subzonal 

 membrane, and therefore with the external epiblast 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 

 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 allantoidean trophoblast. 

 (6) But in the hedgehog, rabbit, and some other Eutherian 

 types, as well as in certain Marsupials, there is a mode ot 

 embryonic nutrition between that attained by the epiblastic 

 trophoblast and that affected 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 con- 

 nected 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. 397), 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, 



