POLYEMBRYONIC DEVELOPMENT IN TATUSIA 579 
are in the act of migrating to the surface. The cells lying within 
the mass (b and d) are rounded in outline, while those in the act of 
coming to the lower surface are invariably elongated in the direc- 
tion of migration. 
The evidence obtained from a study of these sections indicates 
that each group of two or more entodermal cells has, in all proba- 
bility, arisen from a single primary cell, or entodermal mother- 
cell (using Hill’s terminology). Futhermore it suggests that the 
primary cell may, from the first, be situated on the surface; and 
consequently will undergo no migration; or it may migrate to the 
surface without having undergone division; or again, divisions 
may have come in before the migration occurs. 
Blastocyst No. 296 came from the proximal part of the left fallo- 
pian tube, and measured 0.376 mm. in diameter. The embryonic 
spot had a very even outline and measured 0.161 mm. across. 
It is distinctly thicker than the last specimen, averaging about 4 
cells, exclusive of the trophoblast (fig. 10). The are covered by 
the embryonic spot is 50°, 42’. 7 
In every way the blastocyst is more advanced than any we have 
so far described, It is not only larger, but also shows a higher 
state of differentiation. One feature in particular, although not 
entirely unique since it has been observed in one or two other cases, 
is nevertheless worthy of mention. This is the presence of faint 
structures on the outer surface of the trophoblast (fig. 37). These 
were observed in the preserved egg and were naturally taken to be 
the follicle cells, which sometimes adhere to the ovulated mamma- 
lian egg and persist for some time; but in section they are seen to 
be protrusions or exudations from the trophoblastic cells, and are 
probably formed at the time the egg is fixed. 
The number of entodermal cells in the median section is thirteen 
as against sixteen ectodermal cells. This would seem to indicate 
that there has been a great increase of entodermal cells, but in 
several of the other sections they are very much less numerous, 
owing to the fact that the median section passes through the prin- 
cipal groups of entodermal cells. 
The difference in size between the ectodermal and entodermal 
cells is very obvious (fig. 10), and, as compared with the preceding 
figures, stands in sharp contrast. 
