STUDIES IN MAMMALIAN EMBRYOLOGY. Sa 
If this early vascularisation has in this way been brought 
about, it is only natural that the vitelline circulation which 
would occupy the layer below the chorionic circulation—the 
latter having come to spread in the somatic mesoblast as was 
just described—and being thus less effectual in bringing about 
material exchanges with the maternal blood circulating in 
trophoblastic lacune, should- be arrested in its development, . 
the whole yolk-sac participating in this fate. 
Such a process, which would serve to explain the fact which 
we encounter in the early development of man, viz. that a small 
yolk-sac, communicating with the embryonic intestine by a 
wide “yolk-navel”’ (Dotternabel), is separated by a very wide 
interval from the wall of the blastocyst (serous envelope), is 
again rendered more probable by certain details of the hedge- 
hog’s vitelline development. We have seen that in the latter the 
' vitelline circulation does play a very important part in the nutri- 
tion of the embryo, and that large villi, loosely inserted in the 
trophoblast and its lacunary spaces, considerably facilitate this 
process. “But at the same time we noticed that at a certain 
stage of the development this process was arrested, the yolk- 
sac enucleated from the trophoblast and reduced to a folded 
appendage situated against the belly of the foetus. We need 
only picture to ourselves that this process occurs ever so much 
earlier in man—on the grounds developed above—and then we 
will have understood that in reality the two processes are 
directly comparable. If, then, we might assume that the hypo- 
blastic vesicle of the human subject arises in the same way as 
it does in the hedgehog (cf. p. 291, and figs. 22 to 26), and not 
as it does in the rabbit, mole, or other “ macroblastocystic” 
mammals—for which assumption the presence of a true 
well-known “five diagrammatic figures, illustrating the formation of the 
foetal membranes of a mammal,” for which we are indebted to Kolliker, and 
which are also copied in other treatises on Embryology (Balfour, voi. ii, p. 
194), have sometimes impeded instead of furthered an exact insight into these 
processes. The reason lies in the very considerable divergence which is 
noticed in the different mammalian orders, and which does not as yet allow of 
diagrammatic generalisation. 
