DEVELOPMENT OF THE OPOSSUM 63 
with the embryonic area uppermost; the dark spot in the center 
is the rather opaque yolk mass.: 
In the eggs of litter No. 356 the polarity was always apparent 
in whatever medium they were placed, whether in Ringer’s 
solution immediately on removal from the uterus or in alcohol 
after fixation; hence these eggs were readily oriented for section- 
ing. One of these eggs was photographed by transmitted light 
in salt solution and is shown in figure 1, plate 8. It is a perfect 
sphere, situated in the center of the egg as in younger. stages 
(fig. 4, pl. 12; fig. 1, pl. 6). The embryonic area is an opaque 
mass at one pole and the trophoblastic area is a thin layer 
making up the rest of the vesicle. This egg is typical of all 
of this litter (fig. 1, pl. 6), all of which measure 0.17 to 0.20 mm. 
in diameter through the vesicle. Fixation and staining have not 
changed the relation of structures essentially and even the 
distortion due to imbedding is very slight (compare figs. 1 and 2, 
Plo igs >. plo and fig, 1 plo): 
A somewhat transitional stage between Nos. 344 and 356 is 
furnished by litter No. 144 (figs. 1 to 3, pl. 17). These eggs 
were overfixed in Carnoy’s fluid, but are instructive and cor- 
roborative of the trend of development described above. 
In all of these litters (Nos. 144, 344, and 356) the entoderm 
mother cells are still being formed, as the figures in plates 16 
and 17 amply show (ENT”, ENT"). The cells are of the same 
type as those previously encountered, namely, rounded and in 
process of leaving the periphery. Because of the greater 
density of the cytoplasm, these cells often take a deeper 
cytoplasmic stain than do the neighboring cells, from which 
they also become separated by a more definite cell membrane. 
Occasionally the entodermal cells are united into a column, as 
at ENT?, figure 5, plate 17, reminding one of such rows of cells 
in the younger stages (fig. 3, pl. 6). 
In these eggs, too, the margin of the embryonic area seems 
to be the region of greatest proliferation. Thus figure 10, 
plate 17, represents a section near the margin of the area and 
shows a line of primitive entodermal cells; while figure 11, a 
JOURNAL OF MORPHOLOGY, VOL. 32, No. 1 
