DEVELOPMENT OF THE OPOSSUM . 85 
b: The central light field in the embryonic area 
When the diameter of the egg approaches 1.8 mm., certain 
changes of importance have taken place, for in such eggs the first 
proliferation of mesoderm is usually observed. The eggs of 
litters Nos. 189’ and 353’ are of this size; in the last litter meso- 
dermal cells occur and the primitive streak is faintly indicated 
(M, fig. 21, pl. 13); but the other litter lacks a few minutes of 
development to have reached this stage. 
The premesodermal changes in the blastocyst are best illus- 
trated by a typical and favorably sectioned example, namely, 
egg No. 193’ (2), measuring about 1.4 mm. in alcohol. This is 
one of the two eggs shown in figures 1 and 2, plate 10. In surface 
view there is within the embryonic area, a large light field, 
more plainly visible by transmitted light. Such areas have been 
described for other mammalian vesicles of a corresponding stage. 
They are usually somewhat eccentric, sometimes very con- 
siderably nearer one side than the other. I am convinced that 
the point where the light field comes nearest the margin of the 
area markes the posterior portion of the embryonic area, and I | 
have therefore, in figure 9, plate 22, oriented the surface view of 
such an egg with the posterior end down. At a slightly later 
stage, perhaps an hour later, the primitive streak would have 
appeared as a faint tongue-shaped clouding projecting upward 
from the lower margin of the formative area into the light field 
in question, as in the eggs of litter No. 353’, to be described in 
_ the next number of these studies. 
The light field is due entirely to a thinning of the embryonic 
ectoderm (7, fig. 9A, pl. 22), a condition already seen in small 
eggs in figure 6, where at 7’ the area is palpably thinner than at 
either end of the section. At 7, figures 4A and 7, the sections 
also pass favorably to show this central thinner field. In this 
region, too, the nuclei are usually farther separated, whereas at 
the margins they are so crowded as to form a continuous chain 
like a string of beads, although not quite so uniformly arranged. 
