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 aliantoidean 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, 
