STUDIES IN MAMMALIAN RMBRYOLOGY. 321 
actually enter into communication with the trophoblastic spaces 
as to those which are situated at a certain distance from the 
blastocyst. In other parts of the decidual swelling these blood- 
eavities have retained a more flattened endothelium, as is 
usually characteristic for capillaries, and as indicated by letter 
c. in figs. 40 and 41. I cannot say whether the figures here 
given suffice to carry conviction to the reader that, indeed, the 
process by which these endothelia are affected necessarily leads 
to the formation of a layer of bulky cells with conspicuous 
nuclei, immediately surrounding the blastocyst, and at the 
same time enclosing spacious blood lacunz. I have attempted 
to demonstrate that these cells are the modified endothelial 
layer of blood-cavities, which have gradually appeared in the 
interglandular stroma of the uterine mucosa, simultaneously 
with the proliferating process which changes that part of the 
mucosa into the so-called decidua. The blood-spaces are thus 
no ordinary distended capillaries, but formations ad hoc, and 
the vasifactive stroma by which the aforesaid cells are thus 
produced is a structure sui generis. Still it is not the 
cells of the stroma itself, but those by which the blood-cavities 
are more particularly clothed that undergo the metamor- 
phosis here described. 
If the figures here given fail to carry such conviction, I can 
only say that the very numerous preparations which I have 
looked through do not allow of any other interpretation. And 
still more convincing is the comparison with the next stage, 
one section of which was figured sub n° 42, the embryo 
being in the stage between figs. 16 and 17, Pl. XVI. In fig. 42 
little is to be seen of maternal blood-cavities with proliferating 
endothelium. We only detect the trophoblast Tr. of fig. 41 still 
further attenuated, and showing villiform processes, by which 
it is attached to a compact cell-mass Tys., in which a sparse 
cavity is only here and there present. Outside of this cell-mass 
we notice the decidual tissue, in which the decidual stroma D. 
and the blood-vessels can be separately recognised. This 
compact cell-mass Trs. is the product of the further continua- 
tion of the endothelial proliferation, which was above described 
