358 MINOT. [ Vou. IL 
the outer zone of the placenta, a continuous layer of uninucleate 
decidual cells, extending over half the uterus. 
2°. In the ob-placental region the degenerated portion of the 
uterine epithelium is almost completely resorbed around the 
pole opposite the placenta, (compare Figs. 4, 5, and 6). Fig. 6, 
taken from an older stage, in which the phase existing at nine 
days and seventeen hours at the pole is found near the peri-pla- 
centa, illustrates the manner in which the patches of unaltered 
epithelium, g/, of Fig. 5, grew together by the union of their 
edges into a continuous sheet of epithelium, Fig. 6, g/, forming 
a series of shallow cups, widely open. 
3°. The chorion of mammals, as I have defined it elsewhere, is 
“the whole of that portion of the extra-embryonic somatopleure 
which is not concerned in the formation of the amnion.”! The 
term is not applicable until the mesoderm has united with the 
ectoderm in the region outside the embryo to forma single mem- 
brane: such a union has now taken place; the thickened pla- 
cental ectoderm is coated by a thin layer of flat cells, epithelial 
in character and with bulging nuclei. These cells represent the 
lining of the body cavity, or, as this lining is conveniently called, 
mesothelium. The mesothelium, and consequently the ccelom, 
extend a slight distance beyond the edge of the placenta; the 
mesothelium then bends over onto the yolk sack, of which it 
becomes the vascular coat, and then runs ¢owards the embryo; 
the vascular coat has a large vessel, szzus terminalis, near the 
end of the ccelomatic space, and the mesoderm stretches a short 
distance beyond away from the embryo. The ectoderm, on the 
contrary, extends beyond the end of the mesoderm away from 
the embryo over the rest of the yolk sack. Thus the yolk 
sack, as is well known, comprises two parts, one near the em- 
bryo with walls composed of entoderm covered by mesoderm, 
and away from or opposite the embryo, with walls composed of 
entoderm covered by ectoderm ; compare the clearly expressed 
summary of the relations in the rabbit given by Balfour in his 
Comparative Embryology, 11., 199, 200. 
4°. The ectoderm of the embryo presents the same general 
arrangement as at nine days. The area of thickened entoderm, 
however, which is attached to the placenta has changed in ap- 
pearance ; at nine days and three hours each cell outline was 
1 Wood's Reference Handbook of the Medical Sciences, I1., 143; article, Chorion. 
