DEVELOPMENT OF THE OPOSSUM 69 
plate 18, in which certain cells seemed to migrate from the 
embryonic ectoderm to take their place among the entodermal 
cells. I interpreted these cells as entoderm mother cells and 
presented a number of cases which closely parallel the process of 
entoderm formation in Dasyurus as described by Hill. Indeed, 
Hill states that in Macropus the primitive entodermal cells are 
already recognizable as cells situated internally in the blastocyst 
of 0.35 mm. and in Parameles in vesicles of about 1 mm., which 
would seem to correspond very well with the condition in the 
opossum. Certain cells drawn in figures 3 and 4, plate 18, 
might conceivably be proliferating entoderm. If this be true, 
then certainly these sporadic cases are the last stragglers in the 
stream of entodermal cells which arise from the entectoderm. 
The climax in the formation of entoderm in the opossum, how- 
ever, occurs long before this, namely, in blastocysts between 
0.15 and 0.30 mm. in diameter. 
The differentiation of primitive entodermal cells into definitive 
entoderm takes place with rapidity soon after a diameter of 
0.34 mm. is attained (litter No. 194’), so that in vesicles of 
about 0.50 mm. the entoderm has largely assumed its squamous 
structures and lies closely appressed against the simple uni- 
laminar ectoderm. As soon as this differentiation is well under 
way, the entoderm at once migrates beyond its region of origin 
toward the opposite pole of the vesicle. These changes are 
readily observed in some typical examples furnished from litters 
Nos. 194’, 349, 40, 48, 175’, 339, 299’, and 347. 
In litter No. 194’ as described above, the spreading of the 
entoderm has scarcely begun. In litter No. 349 (fig. 12, pl. 17) 
a few cells have flattened decidedly, while the majority are still 
in the condition of indifferent primitive entoderm. The tendency 
to spread is exhibited on the entire margin of the area. As soon 
as the entodermal cells have differentiated, they at once stain 
much darker, a characteristic which they maintain in sharp con- 
trast to the ectoderm throughout the bilaminar stage; this is 
true without exception. 
A somewhat later stage is represented by egg No. 43 (7), 
which is large, and has a greatly attenuated trophoblastic area, 
