DEVELOPMENT OF THE OPOSSUM 67 
sequel will show. At this stage there is no regular arrangement 
of entodermal cells into an epithelium, and, even in the super- 
ficial layer, regularity is only approximated (figs. 13 to 15, 
pl. 17). The cells which are not in contact with the albumen 
are as irregular in shape and size, at least in my specimens, as 
they aré in arrangement; only the nuclei preserve a uniformity 
of size and structure. 
The superficial cells for the most part are clearly embryonic 
ectoderm, and all of the nuclei seen below this layer are primitive 
entoderm. Most of them possess rounded nuclei, and only 
here and there in the sections is there any indication of cells 
which tend to flatten out into definitive entodermal cells (HN T?, 
fig. 15). In this respect there has been little progress since the 
preceding stage. 
Litter No. 194’, was found four days after copulation or about 
two days after the beginning of cleavage. 
A somewhat more advanced stage is represented by litter 
No. 349, one of which is shown photographed in the living state 
in figure 3, plate 8. It measures 0.352 mm. through the vesicle. 
Figure 4 is a section through the youngest egg of the litter and 
belongs to an earlier stage corresponding to litter No. 344. 
One of the two eggs like the one in figure 3 was sectioned, the other 
was accidently broken and was used for study in toto. The 
two are in essential agreement. The formative area, shown as 
a distinct opacity in the living egg (fig. 3, pl. 8), is larger in 
area than in the litter just described; the trophoblastic area is 
thick-walled and less extended than would be expected at this 
stage. The embryonic area is crowded, the cells being three 
and four cells deep in some places. A large number of cells of 
all types—embryonic ectoderm, primitive and definitive ento- 
derm—are in mitosis, chiefly in the spireme stage, as though a 
wave of cell division had spread over the entire area. Here 
and there an entoderm mother cell is still in process of forma- 
tion. The definitive entoderm has begun to differentiate and to 
spread beyond the area (YNT). In surface view the embryonic 
area is approximately round and is sharply marked off from 
the surrounding trophoblastic ectoderm. ‘This description shows 
