DEVELOPMENT OF THE OPOSSUM Co 
entoderm, whereas in the Dasyurus it precedes as well as follows 
the process. At first the blastocyst grows faster than the shell 
membrane, for gradually the albumen disappears before the 
advancing trophoblastic area. This growth would seem, there- 
fore, to affect the trophoblastic more than the embryonic area, — 
which latter lies in contact with the shell membrane from the 
earliest bilaminar stage. The embryonic area, however, easily 
keeps pace with or even exceeds the rate of growth of the entire 
vesicle; for in the 1-mm. eggs it is proportionally larger than 
in certain younger stages. It would seem, then, that, despite 
the absence of albumen, the area receives sufficient nutriment 
for vigorous growth or is indeed better supplied by virtue of its 
superficial position in the egg, with the secretion of the uterine 
glands. 
Eggs about 0.8 mm. in diameter, as illustrated by litter 
No. 306’, removed four and one-half days after the beginning of 
cleavage, still contain considerable albumen which is readily 
visible at all positions of the eggs and may be seen on the pho- 
tographs of the eggs (fig. 7, pl. 10; fig. 17, pl. 13; fig. 1, pl. 21). 
When the diameter of 1 mm. is reached, the quantity of albumen 
has been -considerably reduced and is. mostly confined to the 
trophoblastic region below the equator of the egg (fig. 2, pl. 21). 
In living specimens of such eggs the albumen is visible as a 
narrow crescent, only when viewed from the side, and is not 
visible in photographs of living eggs (fig. 5, pl. 2). The re- 
duction of albumen continues, and when the diameter of the 
egg approaches 2 mm. and the mesodermal proliferation is 
about to begin there is only a thin film of albumen left (fig. 6, 
pl. 2; fig. 20, pl. 13). The amount of albumen at any stage is, 
of course, variable. It may still occur in small amounts in early 
primitive-streak stages (fig. 4, pl. 2; fig. 22, pl. 13). 
The opossum blastocyst, therefore, begins as a perfect spheer 
at about the 32-celled stage. It maintains this shape until the 
_ definitive entoderm begins to spread, when the blastocyst as- 
sumes a biconvex form, flattened in the direction of the egg axis, 
and lies with the formative area against the shell membrane. 
The spherical form is again attained when the trophoblastic area 
