No. 3.] UTERUS AND EMBRYO. 367 
sack expands over the ob-placenta. The cavity of the allantois, 
all, is of course lined by entoderm; it is, however, quite small, 
and in my preparations by no means the spacious vesicle com- 
monly represented, for instance, by Kolliker in his Grundriss 
(2te Aufl. Fig. 88), or by Balfour (Comparative Embryology, 
II., Fig. 148). The allantoic mesoderm, mes; on the other 
hand, spreads out, over the surface of the placenta, down its 
sides, down into the fissure, 4, between the two lobes, and pene- 
trates between the glands, g/, of the placenta; wherever it goes, 
the mesoderm carries foetal blood-vessels. The free, z.¢., inner 
or ccelomatic, surface of the mesoderm bears the mesothelium, 
msth; as the extra-embryonic ccelom does not extend beyond 
the top of the placenta, there is, of course, no mesothelium 
upon the sides of the glandular zone (between a and 8), but at 
the edge of the top of the placenta the mesothelium is re- 
flected back, and after a short course joins the wall of the yolk 
sack near the szzus terminalis, v.t. 
The yolk sack, as has been long known, consists of two parts :} 
Ist, the avea vasculosa bounded approximately by the szxus 
terminalis, vt; within this area the entoderm is united with 
the mesoderm, which passes only a very short distance further 
out ; 2d, the remaining portion without mesoderm, excepting 
always the pro-amnion, which is included in the avea vasculosa ; 
over this second region the entoderm, ez, rests directly upon 
the outer germ-layer, ec/o. 
If we follow the ectoderm around, we find that it leaves the 
yolk sack, just before the szzus terminalis, vt, is reached, and 
being joined by the mesodermic lining of the coelom passes 
down 6 on to the lateral surface between the peri-placenta, P, 
and the glandular placenta, g/, where, as already described, it 
bends inwards, and turning back runs a minute distance up- 
wards ; according to my hypothesis it continued earlier over the 
surface of the placenta, as indicated by the broad dotted line, @. 
The layer of embryonic epithelium upon the side wall of the 
rodent placenta has been seen by other observers, among whom 
may be mentioned Ercolani and Creighton ; the latter, 77b, 560, 
directs especial attention to it, in the Guinea pig, but refers it 
to the entoderm. I consider it probable that it is really ecto- 
dermal in the Guinea pig, as in the rabbit. Underneath the 
1 Leaving the pro-amnion out of consideration. 
