588 J. T. PATTERSON 
cavity between the Traiger and the ectoderm. Material with 
which to follow the steps through which the blastocyst passes 
during this is not at hand, for there is here a slight gap in the 
series. However, I have been fortunate enough to obtain two 
blastocysts which show the condition immediately following the 
inclusion of the ectoderm. The two specimens are nearly of the 
same age, one being slightly more advanced than the other. 
The younger blastocyst (No. 316) was found attached to a 
small leaf-like outgrowth from one of the folds of the placental 
mucosa. The outgrowth lay horizontal to the surface of the mu- 
cosa, and the vesicle had gained attachment to its under side, 
close to the edge. One side of the trophoblastic wall caved in 
during fixation, otherwise the vesicle is in an excellent state of 
preservation (fig. 16). 
That portion of the trophoblastic wall which has caved in pre- 
sents nothing different from what was observed in earlier stages, 
except that the cells are more attenuated, but the opposite wall of 
the vesicle has undergone a marked thickening, in addition to a 
great increase in its cellular elements. The cells have, therefore, 
changed in shape from a flattened squamous type to a distinctly 
columnar epithelium. All the thickened portion of the wall was 
originally in contact with the mucosa, but during the process of 
fixation the leaf-like fold of the mucosa has undergone a great deal 
of contraction and has shrunk away from the trophoblast, as is 
evident from its folded appearance. Evidently the thickened 
trophoblast owes its existence to contact with the mucosa, which 
in some way specifically stimulates the cells to a striking activity, 
as shown by the rate of division and change in shape. The spe- 
cific reaction between the trophoblast and the mucosa must be 
limited to that portion of the uterine epithelium which covers the 
placental area, otherwise the wall of a free vesicle would give evi- 
dence of response before the blastocyst had reached this area. 
It is also a point worthy of note that the columnar cells send 
out pseudopod-like processes, which eat into the mucosa, giving 
it a serrated appearance in section. That the placental tropho- 
blast has therefore a specific action on the trophoblastic cells can 
