302 A. A. W. HUBRECHT. 
the villi attain to a much larger size than is represented in this 
figure, shapes of inverted cones, cubes, &c., and being far 
from rare. The villi are so large and prominent (larger than 
those hereafter to be noticed for the allantois), that when the 
yolk-sac is peeled out of the trophoblast they are easily visible 
to the naked eye. They reach downwards as far as the area 
vasculosa extends. There is no sinus terminalis on the hedge- 
hog’s area vasculosa, although one large vessel, which may have 
misled Fleischmann (lL. c. p. 50), certainly occupies the situation 
where we would expect a sinus terminalis. As, however, from 
this vessel not only centripetal vessels towards the embryo, but 
also centrifugal ones, are seen to emerge, it cannot be called a 
sinus terminalis in the ordinary sense. 
Most curious is the change which is brought about in this 
villiferous vascular region of the yolk-sac, as gradually the 
embryo increases in size, and as the allantois spreads out 
against the dorsomedian diplotrophoblast. 
There is no doubt but that the yolk-sac circulation increases 
for a considerable time, and is of the highest importance to 
the embryo. A time comes, however, at which the embryo 
and amnion grow larger at a much greater speed than the 
whole blastocyst. The yolk-sac is then pushed inwards (ef. 
diagrams 33—35). In consequence of this a larger and larger 
portion of the villiferous area vasculosa is gradually peeled out 
of its trophoblastic surrounding, with which these villi have 
not been so intimately soldered that the process here alluded to 
is in any way impeded. Fig. 44 gives evidence of the process ; 
very numerous other sections are still more demonstrative, and 
show the hollow spaces of trophoblast into which correspond- 
ing omphaloidean villi have fitted. The latter are still in the 
immediate vicinity, but have been scaled out by the increasing 
amnion. Another phenomenon, no doubt, accelerates this 
enucleating process, viz. the fact that the maternal tissue out- 
side the trophoblast, which will be described in the next 
chapter, thins out very considerably all over the region of the 
omphaloidean trophoblast. No such decrease is noticed in the 
region of the allantoidean trophoblast. But with the gradual 
