DEVELOPMENT OF THE OPOSSUM 87 
they are thicker and uniformly granular, but the cell boundaries 
are not as apparent as those of the large flat trophoblastic cells. 
The nuclei of the latter are flatter, but of uniform roundness, 
unchanged by mutual pressure, and possess fewer chromatin 
granules and larger light spaces than the embryonic nuclei, 
otherwise the nuclei of the two areas are very much alike. The 
entodermal nuclei, as a rule, appear larger in surface view than 
those of the ectoderm and they possess a more uniform 
granulation. 
It should be noted that the embryonic ectoderm is still a 
single layer of cells with the nuclei mostly at nearly the same level. 
In the corresponding stage of other mammals the embryonic 
area is considerably thickened as in a pseudostratified epithelium 
(rat, bat). This simple arrangement has the decided advantage 
for the observer in that the very first mesodermal nuclei which 
drop down out of the ectoderm may be located instantly and 
with certainty. 
e. Yolk spherules in ectoderm and entoderm 
In many surface views of bilaminar blastocysts round dark 
objects, usually as large as a nucleus or smaller, frequently meet 
the eye. Sometimes these bodies stain like the cytoplasm, or 
they may be much darker in preparations fixed in osmic acid. 
They are found in both ectoderm and entoderm (figs. 1A and 2A, 
pl. 21; fig. 10, pl. 22), sometimes free, sometimes within the 
cytoplasm and partly enveloped by the nucleus. The inclusions 
are often surrounded by a light zone as though they were partly 
digested and absorbed (fig. 3B, pl. 20); indeed, vacuoles, instead 
of solid masses, in similar situations are not uncommon (V, 
figs. 11 and 12B;pl. 22). 
Blastocyst No. 189’ (12) is worthy of special notice. It 
appeared to be normal in every respect, and the embryonic 
area, which measures 0.75 mm., was dissected off and stained 
and mounted intact in balsam. The interesting feature of this 
vesicle is the large number of these dark bodies that are mostly 
observed in connection with the entodermal cells. The majority 
of these cells underlying the embryonic area are each provided 
