DEVELOPMENT OF THE OPOSSUM Ot 
2. The albumen. The albumen is laid down in delicate, con- 
centric lamellae around the ovum (pl. 13 and others). In the 
living state it is usually of nearly the same density throughout 
the layer (fig. 2, pl. 4) or it may be more concentrated about the 
ovum in young eggs (figs. 3 and 4, pl. 5). 
At first the albumen is extremely poor in protein content, 
for on fixation it usually gathers immediately about the ovum 
and the thin and delicate shell membrane collapses and follows 
close upon the albumen (fig. 5, pl. 5). This phenomenon is 
apparent on comparing plate 12, which represents the living 
condition, with plate 138, on which corresponding stages are 
shown from sections on the slide. The shrinkage of the ovum 
proper or the whole egg in later stages is comparatively slight; 
only the albumen of the younger egg suffers great collapse. It 
follows, therefore, that the albumen layer gradually increases its 
protein content (figs. 14 and 17, pl. 13), and the shell membrane 
likewise grows in thickness and resistance. The uterine ‘milk’ 
doubtless supplies the material thus absorbed. This holds true 
for unfertilized eggs also, which continue to grow in diameter 
and in density of albumen and shell membrane for a week or 
more. Figures on the thickness of the shell membrane have 
previously been given (Hartman, ’16) and are not repeated here. 
It is subject to great variation, as may be seen from the various 
drawings in the plates, where the shell membrane is represented 
in correct proportions. | 
3. The unsegmented ovum. Unless insemination has taken 
place, the uterine differs from the tubal ovum only in the pos- 
session of completed albumen and shel envelopes (fig. 19, 
pl. 14). My collection contains a number of litters of such eggs. 
The first polar body and the second maturation spindle are as 
in the tubal ova, although in some case the chromosomes begin 
to show a clumping and are surrounded by a light area. The 
chromosomes fragment sooner or later, however, and the chro- 
matin breaks up and rearranges itself into round lumps simu- 
lating nuclei in the resting stage. The cytoplasm breaks up 
also, some fragments taking one or several ‘nuclei,’ others none. 
Sometimes the fragments are equal or nearly equal in size, so 
