590 J. T. PATTERSON 
subsequently become lined with a layer of mesoderm. The con- 
- dition here presented reminds one somewhat of that found in 
a corresponding stage of the blastocyst of Pteropus edulis 
(Selenka and Goéhre ’92), except that in the case of the latter the 
entoderm continues around the inside of the trophoblast, forming 
a complete, inner layer to the blastocyst. 
In the older of the two blastoeysts (No. 332) the sections pass 
‘exactly parallel to the median axis of the vesicle, and consequently 
the relation of the different parts of the embryo is clearer than 
in No. 316. The general conditions of the trophoblast are much 
the same in the two specimens, except that at one point on No. 
332 there is a knot of cells (fig. 17, &) which, in the living condi- 
tion, fitted into a corresponding crypt in the mucosa. In later 
stages we shall see further evidence of similar knots, which rep- 
resent points on the trophoblast that have been specifically 
stimulated to cell proliferation. 
The relation of the entoderm to the embryonic ectoderm is re- 
markably clear in this preparation (fig. 17). In contrast with 
the preceding blastocyst, the entoderm of this specimen has under- 
gone one important change, in that it has folded in beneath the 
ectoderm, forming all but a closed entodermal sac. Only a small 
pore-like opening (fig. 17) remains to place the extraembryonic 
cavity in communication with the cavity of the entodermal sac. 
On the right-hand side (or lower side, owing to the inclination to 
the right of the blastocyst) a fusion has taken place between the two 
layers of the entoderm and a portion of the Trager; but this fu- 
sion, especially with the Trager, covers a very small area, as it no 
longer exists in the sections a short distance to either side of this 
one. The loop of entoderm which lies between the fused area and 
the pore (fig. 49) is probably comparable to the group of cells sit- 
‘uated in a similar position in the other specimen. The loop of 
cells is especially clear in figure 17. 
Returning to a fuller consideration of the ectodermal sphere, 
we see that even in so young a stage as that of No. 316 it is no 
longer a solid mass of cells, as must have been the case at first, but 
in the central portion there are three relatively large and distinct 
besides several smaller, less distinct vacuoles. In fact, the 
