582 J. T. PATTERSON 
of about 0.4830 mm. At this time the embryonic spot has not 
reached its maximum expansion, that is, it has not completely 
flattened out. Consequently in section the entoderm forms a 
slightly curved line, due to the bulging out of the mass of embry- 
onic ectoderm (fig. 14). The ectodermal cells are large, relatively 
clear, and sharply cut off from the overlying trophoblastic cells. 
On the inner surface of the mass they are less sharply separated 
from the entodermal cells, which here and there send processes up 
between some of the bordering ectodermal cells. Such processes 
are undoubtedly the last remnants of the migrating entodermal 
cells to be withdrawn from the embryonic mass. 
‘The condition of the entoderm is of much interest. It does not 
as yet form a complete sheet of cells underlying and coextensive 
with the embryonal ectoderm. In places the entoderm may be 
wanting for more than the width of a cell. Furthermore, on the 
right side of the sections of specimen No. 249 the entoderm is fre- 
quently wanting, indicating that it must have taken its origin from 
the left portion of the embryonal mass. In some of the earlier 
blastocysts this same fact was observed. The smaller, deeply 
staining cells, which give rise to the entoderm, were found to ex- 
tend over not more than two-thirds of the embryonic mass. 
Finally, in the largest free blastocyst secured (No. 300) and one in 
which the entoderm is completely segregated, we find this same 
excentricity of the entoderm. It was so evident in this specimen 
that I have gone to the trouble of making a special preparation 
for the purpose of demonstration by a photograph. The vesicle 
in question was slightly stained in eosin and imbedded in paraffin. 
Under the high power of the binocular microscope the non-em- 
bryonic hemisphere was carefully pared away with a sharp razor. 
The remaining hemisphere was dissolved out of the paraffin, 
stained in hematoxylin, and mounted in balsam, with the cut sur- 
face uppermost. Thus it was possible to get an unobstructed 
view of the entoderm, and since the embryonic mass had become 
completely flattened, the entodermal layer lies in a single plane. 
The exposed sheet of entoderm was then studied and pho- 
tographed (fig. 42). 
