354 MINOT. [VoL. II. 
lead rather to the supposition that the embryo normally lies 
across the uterus; but this is true in my experience only of the 
later stages after the embryo is suspended more freely. 
Next as to the extra-embryonic portions of the germ layers: 
my observations compel me to differ as to the composition of the 
walls of the blastodermic vesicle or yolk sack. Sections through 
the middle region of the embryo, Cut 1, show an open medul- 
lary groove, JZd, the thickened notochordal band, zch, of the 
entoderm; the mesoderm of the Stammzone is undivided ; that 
of the parietal zone is split to form the ccelom, Coe, which may 
be followed a considerable distance from the axis of the embryo, 
when the two leaves of the mesoderm again unite into a single 
distal plate, mes, in which the two layers can be distinguished 
for a certain distance; the distal edge of the mesoderm is 
sharply marked; the mesoderm is thickened around its margin. 
Beyond the margin the ectoderm and entoderm come into con- 
tact, a; the former is the thicker layer, being composed of cubical 
cells, while the entoderm consists of very thin broad cells; the 
two layers continue outwards until, having passed over the peri- 
placental thickening, they reach the region of transition from the 
peri-placental to the ob-placental area, where the ectoderm chang- 
ing to a thin flat-celled membrane is intimately conjoined to the 
degenerated epithelium of the uterus. Where the concrescence 
takes place, the union of the two layers becomes very intimate, 
so that it is difficult to satisfy one’s self how much farther the 
ectoderm extends, but apparently it goes completely around, 
forming a closed vesicle, as is generally stated and as is found 
later. Apparently the ectoderm is not involved in the resorption 
of the uterine epithelium, which disappears — but further inves- 
tigation is required. The entoderm of the other hand is readily 
followed, and although somewhat crumpled and torn in the ob- 
placental region of my specimens, there is to my mind very little 
doubt that it forms a closed sack, corresponding to the ento- 
dermic lining of the yolk sack of other mammals. From the 
observations above recorded it is evident that two germ layers 
are readily traced for some distance from the embryo, but that 
beyond a certain line only one layer, the entoderm, is readily 
followed. Now this fact is carefully represented in van Bene- 
den’s and Julin’s diagram, 44, Pl. XXIV., but the membrane 
which stops is there given as entoderm, which, as seen in my 
