DEVELOPMENT OF THE OPOSSUM 45 
given by the authors in their figures 12, 13, and 14 I recognize 
from my experience with hundreds like them as typical pictures 
of degenerating eggs; for example, in the thickness of the shell, 
which suffers little collapse in fixation; in the peculiar stringy, 
not uniformly concentric character of the albumen, and in the 
character of the ovum itself, where fragmentation is quite ap- 
parent. 6) Finally, the size of the eggs as stated by the authors, 
0.75 to 1.5 mm., is far above that of normal eggs in cleavage 
and entirely in agreement with my own specimens of fragmenting 
eggs. I must, therefore, conclude that the eggs described by 
Spurgeon and Brooks do not represent normal cleavage in the 
opossum. 
I would conclude, therefore, that both blastomeres of the 
2-celled opossum egg divide meridionally, but that they shift 
their position during division so that the resulting 4-celled ovum 
possesses the typical crossed arrangement. ; 
g. Comparison of the 4-celled egg of the opossum and of Dasyurus 
It is seen from the foregoing that the 4-celled stage of the 
opossum is typically Eutherian, at least in the arrangement of 
the blastomeres, and quite different in every recognizable way 
from the egg of Dasyurus, in which, as described in Hill’s 
beautiful monograph, the second cleavage is shown to be 
meridional, dividing the egg into four equal cells which exhibit 
the same polar differentiation as the 2-celled egg, for each 
blastomere possesses a larger, vegetative pole and a smaller, 
relatively yolk-free animal pole. 
Precisely such an egg is described by Selenka (’87) for the 
4-celled stage of the opossum. I can reafflrm my former state- 
ment that this is a case of an unfertilized egg undergoing pseudo- 
segmentation or amitotic fragmentation, in which the four 
pieces or ‘blastomeres’ (pseudoblastomeres) happen to be of 
equal size. I have seen such eggs dozens of times. Figure 5, 
plate 11, is a photograph of a litter of eggs, palpably frag- 
menting, but showing one ‘2-celled’ and one ‘4-celled’ stage 
which might easily be mistaken for normal cleavage. ‘This 
