DEVELOPMENT OF THE OPOSSUM 65 
cysts last described, the superficial cells at the upper pole are 
ectodermal except a few which are destined to form entoderm 
mother cells. 
j. The primitive entoderm 
In the stages thus far described the entodermal cells are still 
round to polygonal and only occasionally does a cell flatten 
out upon the surface of the mass as at LNT", figure 4, plate 17. 
We may call these cells primitive entodermal cells (HNT?) as 
distinguished, on the one hand, from the large entoderm mother 
cells from which they arose (HN7") and on the other, from the 
typical, flattened definitive entoderm into which they are about 
to develop. The primitive entoderm, through rapid cell division, 
becomes more or less crowded and shows a tendency to become 
two or three cells deep, as early as the stage represented by 
litter No. 356 (pl. 17). 
k. Further growth of the blastocyst 
When the blastocyst contains less than 200 cells, of which 
about 20 would be entodermal (litter No. 344, pl. 16), the 
embryonic and the trophoblastic areas each make up about 
one-half of the blastocyst wall. When the number approaches 
300, including 40 or 50 entodermal cells (litter No. 356, pl. 17), 
the latter area has greatly extended so that the embryonic 
portion occupies a third or less of the blastocyst wall. The 
increase in size of the blastocyst is, therefore, to be attributed 
largely to the spreading and attenuation as well as a more 
rapid multiplication of trophoblastic cells (table 6). 
The eggs of litter No. 194’ are illustrative of the further 
development in the direction just indicated and follow close 
upon the eggs of litter No. 356. The vesicle has grown from 
about 0.23 mm. in diameter as the maximum for litter No. 356 
to 0.34 mm. in litter No. 194’, or about double the diameter of 
the ovum at cleavage. The blastocyst is still situated in the 
center of the egg, which has, however, not yet increased in 
