DEVELOPMENT OF THE OPOSSUM 79 
tinuous, because in the fixing fluid portions of the cells break 
down. That this is the correct explanation is seen from surface 
views of good preparations, as in figure 11, plate 19, in which only 
the entodermal cells are shaded. They are seen to be connected 
by fibrous strands, the coagulated portions of the delicate cells. 
The entoderm may, therefore, be considered practically con- 
tinuous. In most surface mounts the entoderm appears like a 
mottled surface, for the cells are thick in the middle, hence 
darker, and shade off almost into nothingness toward their 
edges (fig. 2, pl. 19). 
The entoderm invariably stains darker than the ectoderm. 
It is interesting to note that an entodermal cell in mitosis is 
darker, often very decidedly so, than its fellows in the resting 
stage (fig. 2, pl. 19); while on the other hand an ectodermal cell 
is usually much lighter when in mitosis (fig. 12, pl. 19), so that 
in some surface views dividing cells look like holes in the wall. 
Yolk granules abound among the cells of the embryonic area, 
both ectodermal and entodermal, and occur occasionally also in 
the trophoblastic region. 
A somewhat older egg (figs. 4 and 4A, pl. 20) is presented be- 
cause of the degenerating cells included within the cavity. 
Such cellular remnants have been noted in apparently normal 
- vesicles of Eutheria (Hartman, ’16, pp. 46 and 47). In this egg, 
too, the albumen is definitely arranged in three layers of varying 
density, a condition noted also in a few other specimens. 
THE 1 MM. BLASTOCYST 
a. General description 
The typical 1-mm. blastocysts contained in litter No. 343. 
(fig. 6, pl. 2) were removed seven and a half days after copu- 
lation. It has already. been pointed out that only a small 
amount of albumen still remains in these eggs and the vesicle 
has become very nearly a perfect sphere. The embryonic area 
is now more sharply marked off from the surrounding tropho- 
blast and lies like a cap at one pole of the egg (fig. 9, pl. 21). 
It is, in fact, occasionally in alcoholic specimens raised in relief 
JOURNAL OF MORPHOLOGY, VOL. 32, NO. I 
