yy CARL G. HARTMAN 
tive, that a part of each type of cell goes into the formative and 
a part into the non-formative region. 
Because of the short period in the opossum egg in which polar 
differences are lost, it is, therefore, impossible to demonstrate 
cell lineage in the cleavage of the opossum egg. But the same 
statement may be made with reference to the egg of Dasyurus, 
as pointed out by Hill himself, for, in the Dasyurus blastocyst, 
polar differentiation is lost during the long period of growth 
from 0.6 to 3.5mm. Hill says (’11, p. 46): “It might therefore 
be supposed that the polarity, which is recognized in early blas- 
tocysts, and which is dependent on the pronounced differences 
existent between the cells of the upper and lower rings of the 
16-celled stage, is of no fundamental importance, since it ap- 
parently becomes lost at an early period during the growth of 
the blastocyst. Such an assumption, however, would be very 
wide of the mark. . . . . and, indeed, in view of the facts 
set forth, is an altogether improbable one.” There is not the 
slightest doubt that Professor Hill’s view is the reasonable and 
the probable one. Upon the same grounds, the 2-celled opossum 
egg is not homologous with the 2-celled egg of Dasyurus, but 
rather with the 2-celled Eutherian egg. 
Several abnormal opossum eggs are instructive in this con- 
nection, for they are indicative of polar differentiation in young 
blastocysts normally devoid of evidences of polarity. They are 
abortive attempts on the part of one-half of the egg in each 
case to form a blastocyst wall and they were found in litters 
of eggs made up for the most part of normal young blastocysts. 
Figure 3, plate 16, represents a section through an egg in which 
one-half of the blastocyst, consists of twenty cells, which have 
flattened normally, whereas the other half consists of eleven 
cells which are still rounded as in an earlier cleavage. Similarly, 
one-half of another egg (fig. 4, pl. 16) seemed to develop normally, 
the other half containing cells with fragmenting nuclei. In still 
another the normal half is beehive-shaped and surrounds two 
very large and three small cells (fig. 10, pl. 21). Egg No. 356 
(2) has two large blastomeres at one pole of the blastocyst 
. (fig. 14, pl. 22) and egg No. 88 (6) has a retarded blastomere 
enclosed within the blastocoele (fig. 18, pl. 22). 
