STUDIES IN MAMMALIAN EMBRYOLOGY. 317 
nent part and which commences about at the same time at the 
bottom of the groove in which the early embryo is situated. 
It is to this phenomenon, by which the definite attachment 
of the embryo to the mother is brought about, that we have 
now to direct our attention. 
We have left the young blastocyst at the bottom of the 
groove (Pl. XV, fig. 5) as yet unattached to maternal tissue. 
The epithelium of the groove is still very distinct, although 
signs of a retrogressive metamorphosis at different spots are 
not wanting (fig. 37, 1). This retrogressive metamorphosis 
even goes hand in hand with a proliferation of the epithelium- 
cells, often still more clearly visible in a following stage 
(fig. 38). It appears that when the blastocyst comes into 
actual contact with it, the disappearance of the epithelium is 
still further accelerated. At least I have another embryo, taken 
from the same mother as the one that is figured on Pl. XV, 
fig. 5, and in which the embryo is seen to be wedged between 
epithelium-cells and touches the subepithelial stroma with a 
few of its own cells. ‘This is still more definitely the case in 
fig. 38, where one lateral surface of the blastocyst adheres 
against the compact decidual stroma without any remnant of 
the uterine epithelium between them, which shortly before 
must have covered that spot quite as much as it does as yet all 
the rest of the lumen /., and as it undoubtedly does in the 
figs. 5 and 37. 
The question immediately presents itself whether it is 
perhaps the direct active influence of the blastocyst itself, i. e. 
whether possibly the outer trophoblast-cells absorb the epithe- 
lium they come into contact with, the blastocyst eating its 
way, if I may use that expression, towards the deeper layer in 
which it is going to be safely and advantageously imbedded. 
It will suffice to have put the question for the present, and to 
postpone a definite answer till the time that the evidence of 
more numerous examples can be brought to bear upon it. 
Selenka has given an affirmative answer to this same ques- 
tion with respect to the mouse’s blastocyst and the uterine 
epithelium-cells with which it comes into contact (Keimblatter 
