No. 3.] UTERUS AND EMBRYO. 409 
cation between the foetal and maternal tissues is here lost, and 
in fact, at the edge of the placenta the decidual cells make their 
way into the chorionic tissue, and for a certain distance towards 
the centre of the placenta they are found lying chiefly in the 
ectoderm. In other placentz the fibrine layer and the decidual 
tissue around the margin of the placenta have not only inter- 
grown, but also undergone a common degeneration, Cut 25, in 
consequence of which all distinct structure is obliterated, and we 
find the villi, vz, imbedded in a stratum, 7, of more or less 
colored substance, without definite organization except irregu- 
larly scattered nuclei. Attentive examination shows that this 
layer, f, has unmistakable remains, c, of the cellular layer next 
the mesoderm of the chorion, and that it passes into an outer 
layer, D', in which the traces of decidual structure are unmistak- 
able ; the dark line at the lower edge of the decidua, D’, is merely 
detritus and coagulum, as is often found on after-births. If we 
follow the layers in this, or a similar specimen, in the direction 
away from the placenta, the layers gradually alter, losing their 
degenerated character, until we reach a point where the 
chorionic ectoderm and the uterine decidua both exhibit their 
normal features. Returning now to the placenta we were pre- 
viously considering, Cut 24: The placental chorion begins to 
exhibit its characteristic stratification a short distance within 
the margin. I have found, however, that the distinctness of 
that stratification varies considerably, not only in different 
placentze, but also in different parts of the same placenta. The 
decidua, D, outside the placenta is very thick, but at the edge 
of the placenta it begins to thin out, and as it passes over the 
under side of the placenta, rapidly becomes so much reduced as 
to be even less in thickness than the chorion, cho. The decidua 
is everywhere crowded with an immense number of decidual 
cells, but in some other specimens they are less crowded. The 
surface of the decidua serotina is covered by a layer of fibrine, 
easily recognized by its deep staining ; this coat of degenerated 
material has not yet received the attention it deserves, as a fea- 
ture of the human placenta, which is quite constant, so far as 
my observations go; as stated previously, I consider its origin 
to be the epithelium of the ends of the villi imbedded in mucosa. 
Up to the edge of the placenta the chorion leve and decidua 
are united; at the edge they separate, to make room for the 
