DEVELOPMENT OF THE OPOSSUM Ta 
The other more abnormal eggs referred to above show even 
at low magnification evidences of abnormality (fig. 2, pl. 6). 
The vesicles are not plump and rounded, but more or less 
shriveled and are surrounded by a large ‘perivitelline space.’ 
That the abnormalities are not due to the method of fixation is 
shown by the appearance of the living eggs, of litter No. 294 
reproduced in figure 1, plate 11. In sections made of these 
eggs the walls are composed of cells swollen to enormous volume 
and are extreme cases of the condition shown in fig. 5A, pl. 
19. Many cells are in mitosis, with the chromosomes strewn 
about pell-mell throughout the cell. Similar eggs are also met 
in normal litters (fig. 4, pl. 9). The ‘pear-shaped vesicle’ de- 
seribed by Selenka and figured in his Tafel XVIII, Fig. 1 u. 2, 
was doubtless an egg of the type just described. 
PART IV. THE BILAMINAR BLASTOCYST 
GENERAL DESCRIPTION 
a. Material 
The various stages in the bilaminar blastocyst of the opossum 
are represented in my collection by an unbroken series separated 
from one another by minutes rather than hours of development. 
Two hundred and thirty-five normal eggs were secured from 
thirty-two litters of twenty-five different animals; hence it 
may be assumed that, the following description gives in detail 
the normal opossum egg during these stages. One hundred and 
fifty eggs were sectioned or were dissected for study of surface 
views; and the former include many that were carried through 
the imbedding and sectioning process without collapse and 
with the minimum of shrinkage. 
b. The living eggs 
The general trend of development during this period may 
be followed by reference to the photographs of living eggs 
presented in plates 1, 2, 9, and 11 of this paper. 
