No. 3.] UTERUS AND EMBRYO. 363 
embryo; it is therefore mesothelium. Underneath this covering, 
and above the glands, there is a layer of varying thickness con- 
taining some large and a few small blood-vessels with embryonic 
blood in them, and consisting otherwise only of scattered anasto- 
mosing connective tissue cells,! which can be followed without 
the slightest break on the one part until they pass directly into 
the mesoderm of the superjacent embryo; on the other part, 
down between the glands, Fig. 8, wes ; compare, also, later stages, 
Figs. 10 and 11. Between the glands, also, are blood-vessels con- 
taining embryonic blood. On the top surface of the placenta I 
can find nothing recognizable as even a trace of the foetal ecto- 
derm, which formed a thick and conspicuous covering in the 
latest previous stage examined (nine days and seventeen hours). 
At the edge of the top of the placenta, Fig. 8, the relations 
change: the mesothelium, ms¢, bends up and leaves the placenta, 
and together with a few subjacent mesodermic cells joins a sheet 
of cylinder epithelium, cto, which is shown by its connections 
to be foetal ectoderm. The ectoderm from the point where the 
mesothelium, mst, bends on to the top of the placenta con- 
tinues downward, c/o, to clothe the side of the placenta which 
faces the peri-placenta. Between the placenta and peri-placenta, 
as shown in Fig. 9, there is a fissure; the ectoderm can be fol- 
lowed to the bottom of this, and from there extends, —not on to 
the peri-placenta, — but turns abruptly back on to the side of the 
placenta, up which it stretches a minute distance and thereupon 
ends abruptly. The disappearance of the ectoderm is discussed 
in the next section. 
The peri-placenta is now characterized by the enormous 
increase of the perivascular decidual cells and the accompany- 
ing expansion of the blood-vessels; by the disappearance of its 
glands and by the reconstitution, in part, of its superficial epi- 
thelium. The peri-placenta appears like the continuation of the 
outer zone of the placenta, for it directly adjoins it, is of about 
the same thickness, and is histologically similar. The blood- 
vessels are wide with hypertrophied endothelium; the peri- 
vascular cells are disposed as in the sub-glandular zone of the 
placenta; that is, in the half towards the uterine muscularis they 
completely fill the intervascular room, but in the half towards 
1A layer closely similar to this, and presumably homologous with it, exists in the 
Guinea pig (Creighton, 77a, p. 558), in the rat (Ercolani), and other rodents. 
