308 A. A. W. HUBRECHT. 
can thus reach the opposite side of the hollow allantoidean sac, 
either more directly along these strands of connective tissue, 
or they can follow the surface and then enter the discoid area 
of placentation at the border. These two methods are actually 
present side by side; and in the ripe embryo, shortly before 
birth, the allantoic vessels connecting embryo and placenta 
are seen to encircle the embryo on one side in a common 
bundle. Behind the embryo’s back the vessels composing this 
bundle separate and run in different directions, some being im- 
planted in the centre of the placenta, some running towards its 
border, there to insert and further to ramify themselves. 
Diagrams 34—86 give indications of the facts here referred to. 
The cavity of the allantois is retained during the whole em- 
bryonic life; the inner hypoblastic lining may for a long time 
be traced, and is also histologically distinct (fig. 54—56, h y. a) 
from the mesoblastic allantoidean tissue. Ramified connective- 
tissue cells and a more hyaline ground-work carrying the 
allantois vessels are there noticed in later stages, as represented 
in figs. 54—56, ail. 
Tt is from the wall, which is applied against the diplotropho- 
blast, that the villiform or ridge-like outgrowths of the allantois 
take their origin, which will fuse more and more completely as 
the age of the embryo advances with the superposed trophoblast. 
In an early stage they are figured on Pl. XXV, fig. 54, and 
seen to be provided with blood-vessels from the outset. 
The trophoblast cells, where they overcap the allantoidean 
villus, are seen to retain a more or less epithelial arrangement, 
and this arrangement is retained during the next phase of 
development (fig. 55). It is even still visible—although some- 
what more indistinectly—in a yet later phase (fig. 56), but in 
the ripening and full-grown placenta this arrangement has 
disappeared, and the fusion of the cell material of the villi with 
the cellular trophoblastic tissue is so complete that it is im- 
possible to draw the boundary line between the two. This can 
be fully appreciated by a glance at fig. 57, which has reference 
to the nearly ripe placenta. 
My preparations show that the villi are radially implanted 
