310 A. A. W. HUBRECHT. 
half consisting of more or less considerably modified maternal 
tissue that will be more fully described in the following 
chapter. 
When the placenta, after parturition is in its turn detached 
and is found free in the uterus-lumen (as it was noticed by me 
on more than one occasion), a section still more clearly reveals 
that difference between the two constituent parts, a difference 
which was known both to Nasse and Ercolani, although the 
real origin and the nature and details of the difference escaped 
their attention. 
The trophoblastic portion of the placenta when the latter has 
thus become a true after-birth is seen in sections to correspond 
to the stage fig. 57; it has acquired the aspect of a still denser 
network of which the meshes are formed of the allantoidean 
and trophoblastic constituents, the spaces between the meshes 
being filled with maternal blood. The after-birth is disc- 
shaped; the foetal membranes are attached to its border (cf. 
fig. 86); against its concave surface the allantois may be 
detected ; its convex surface is modified maternal decidual tissue 
that overcaps the deeper lacunar regions of the placenta in 
which the villi are bathed. 
We have now followed both yolk-sac and allantois through 
their principal phases. We have seen that they enter into very 
intimate interlocking not with any maternal tissue, but with 
purely embryonic cell-material to which we have applied the 
name of trophoblast, and which undergoes different phases of 
increase and decrease in thickness and in extension according 
to the age of the blastocyst and according to the portion of its 
wall which we examine. This trophoblast we have seen to be 
highly lacunar, the lacune to be filled with maternal blood. 
And so it is not the question: how do allantois and yolk-sac 
succeed in approaching vascular regions of the maternal 
uterine tissue? but: (1) How does the trophoblast become 
connected with the maternal tissue long before the appearance 
of either vitelline or allantoidean circulation? (2) What modi- 
fications do the maternal tissues undergo in order to bring 
about the unexpected result that maternal blood freely circu- 
