100 A. A. W. HUBRECHY. 
remaining surface of the blastocyst, which does not adhere to 
the maternal mucosa. 
A second most instructive case of attachment of the 
didelphian blastocyst to the maternal tissue we also owe to 
Hill when he described the early stages of Dasyurus (00). 
He finds the allantoidean diplotrophoblast in full retreat and 
degeneration, the allantois itself hardly vascular and evidently 
abdicating. The contact with the maternal nutritive matter 
is brought about during the eight days of gestation by a 
ring-shaped zone (av, Fig. 150) where the omphaloidean vessels 
form a vascular ring, which undoubtedly facilitates the respira- 
tion of the embryo, while below this ring another ring of 
peculiarly developing trophoblast constitutes still another sur- 
face upon which nutritory processes are inaugurated (atr, Fig. 
150). This lower ring is about one and a half the width of 
the vascularised omphaloidean ring, and is also distinguished 
from the latter by the much more considerable activity 
of the trophoblast cells that form the outer layer of this 
omphaloidean diplotrophoblast. Hill describes in detail how 
the trophoblast cells surround and destroy part of the maternal 
uterine epithelium, how they then reach maternal sub- 
epithelial capillaries, how they envelope these and gradually 
develop into a syncytium of undoubted nutritive significance 
for the embryo. It is interesting to note that at birth the 
embryonic proliferations are not shed (nor is any maternal 
tissue), but that they are absorbed in situ, as I have described 
it for the mole (contradeciduate type of placentation, p. 124). 
2. Monodelphia. 
Passing on to the monodelphian mammals, we find an 
endless variety in the adaptation of the trophoblast to early 
phenomena of adhesion, of nutrition, and of phagocytosis, the 
latter leading up to an actual embedding of the blastocyst in 
maternal tissue, thus ensuring an all the more extensive 
possibility of mutual osmotic interchange between the 
maternal and embryonic vascular systems. 
