C2 CARL G. HARTMAN 
cleavage. This delay is in contrast, again, with the egg of 
Dasyurus, in which the blastomeres in the 16-celled stage have 
already establ’shed contact with the shell membrane on_ all 
sides, the albumen of the egg, always limited in thickness, having 
entirely disappeared. The formative area almost always reaches 
the shell membrane first, the exception being very rare. The 
albumen, therefore, becomes concentrated at one pole, gradually 
decreasing in amount with the growth of the blastocyst. Hence- 
forth the stage of advancement of the blastocyst may be gauged 
by the amount of albumen which appears as a crescent in the 
egg when viewed from the side or when seen in a longitudinal 
section (eggs No. 299’, in figs. 1 and 2, pl. 6; fig. 5, pl. 10, etc.). 
That this eccentric position is not an artifact due to fixation or 
other causes, is shown by the fact that in the living egg the 
blastocysts are situated in exactly the same position as after 
fixation, as the photographs (fig. 4, pl. 1; figs. 4 to 6, pl. 9) 
show. 
p. Some abnormal eggs 
Since future workers on the opossum are likely to encounter 
abnormal material, it is not amiss to describe several abnormal 
eggs of about the stage just described. 
In the group of eggs in figure 2, plate 6, a number of such 
abnormal specimens are shown: Nos. 294 (1), 294 (2) and 294 
(3) and 339 (4). The last mentioned is the least abnormal of 
all. In the living state the vesicle was spherical and remained 
so throughout the process of imbedding (figs. 5 and 5A, pl. 19). 
A similar egg is shown in figure 15, plate 18, and the embryonic 
area of a third in figure 6, plate 19. These eggs, all from one 
litter, are in close agreement and the relation of ectoderm and 
entoderm is as in normal eggs of this stage (compare pl. 18). 
But the wall of the vesicle consists of unduly inflated cells with 
very diffuse cytoplasm and often large nuclei. Egg No. 339 (3) 
is exceptional in this litter, for its normal appearance; it doubt- 
less represents the normal stage to which the others should have 
attained (fig. 6, pl. 9; fig. 2, pl. 6; figs. 6 and 6A, pl. 18). 
