DEVELOPMENT OF THE OPOSSUM Sl 
from it in fixation (fig. 7A). In the 1l-mm. blastocyst it will 
endure fixation better than in younger stages. In all cases, in 
contrast with the uniform granulation of the embryonic area, 
the trophoblastic cells are reticulated and often possess coarse 
meshes or appear highly vacuolated. In extreme cases, es- 
pecially when fixed in aceto-osmic-bichromate, the trophoblastic 
area may be greatly swollen; but this may also sometimes happen 
even in so reliable a fluid as Bouin’s, as in figure 4, plate 22. 
The trophoblastic area is thus much more affected by fixation 
than the embryonic area. 
While, as a rule, the trophoblastic area is rather uniform in 
' thickness throughout its extent, there are frequent exceptions 
which deserve special mention. The area may gradually thicken 
toward the lower pole (fig. 4, pl. 22), or there may be a thick 
mass of cells jutting out into the albumen, and even touching 
the shell membrane at that point. In such cases the entoderm 
is continuous over the mass. In still other cases the ectoderm 
at the extreme lower pole is depressed outward into a pocket 
which may also come into contact with the shell membrane 
(O, figs. 2 and 3, pl. 21). The entoderm bridges over this cavity 
in a continuous layer and does not follow the ectoderm into the 
pocket. In the whole egg the pocket is quite evident and 
looks like a blister on the vesicle. These structures can scarcely 
have any special significance, since they are not of constant 
occurrence, nor are they situated at a point of special future 
importance. 
As in both younger and older stages, the entoderm is a con- 
tinuous layer lining the entire cavity of the blastocyst. It 
consists of a very attenuated layer of large squamous cells quite 
typical of the corresponding stage of all mammals. The cyto- 
plasm is mostly gathered near the center of the cell, where the 
nucleus lies. In surface views the entodermal nuclei usually 
appear larger than the ectodermal and the chromatin granules 
in them are more evenly distributed. They can be recognized 
by this difference as well as by the depth of focus required to 
see them. The entoderm always has a stronger staining 
reaction than the ectoderm; I find no exception to this rule. 
