292 A. A. W. HUBRECHT. 
22). It must be inevitably concluded from these sections that 
at this stage of development the inner hypoblastic cell-mass is 
not compact, but hollowed out towards the middle. The con- 
stituent cells are all of the same character, rounded and not 
flattened. A faint indication of a commencement of a tension, 
by which part of these cells might be flattened, is seen towards 
the left lower corner of the hypoblastic mass in fig. 24. 
From figs. 6, 10, 23, 24, and 25, especially when compared 
to fig. 21 of the earlier stage, I would be inclined to conclude 
that the hypoblastic cells are more particularly in contact with 
the epiblastic wall at a definite point, which is situated a 
short distance below the knob that is going to develop into the 
embryonic epiblast. I would suggest the possibility of this 
spot being directly comparable to what Selenka designates as 
the blastopore of the opossum, a spot which in the full-grown 
didermic blastocyst he is yet able to recognise as lying just 
behind (i.e. somewhat excentrically from) the germinal area. 
This question, together with that whether this blastopore could 
further be compared to that of Amphibia and lower Chordata, 
is, however, too important to be answered either one way or 
the other on data that are as yet so far from being exhaustive. 
Still, I may not omit calling attention to it, all the more 
because in many preparations that are not figured the same 
point of contact between hypoblast and epiblast at these early 
stages is noticed, and is correspondingly situated. When once 
the hypoblastic cell-mass has become a hollow sphere with a 
cell wall that is one cell thick, this hollow sphere does not im- 
mediately fill out the segmentation-cavity. This cavity persists 
for a short time between the epiblast and the hypoblast, both 
of which have the form of closed sacs. Very soon, however, 
the inner hypoblastic sac is closely applied against the epiblastic 
wall, but at the same time a marked differentiation along two 
distinct lines has set in between the constituent hypoblast-cells 
which, in the earlier stages just referred to, were not yet different 
from each other. 
Thus in the preparations figured sub N°: 4—7, 18, and 2] — 
25, the hypoblast cells (which enclose the cavity that will de- 
