DEVELOPMENT OF THE OPOSSUM 47 
dropped bodily. The appearance of these cast-off masses in the 
various stages may be seen from the drawings. With tri- 
chloracetic fixation, blastomeres and yolk blend into an almost 
uniform mass, so that the limits of the cells are recognizable 
with difficulty (compare fig. 13, pl. 15, with fig. 14, eggs from 
the same litter). 
The yolk elimination in marsupials is, of course, striking in 
that the mass involved is very large, and this is as one would 
expect from the phylogenetic position of the group, as has been 
so ably discussed by Professor Hill. This phenomenon has, 
however, not entirely disappeared among the EKutheria, as Van 
der Stricht’s fine study of the bat ovum amply proves. This 
author has shown that there is a polar distribution of the yolk 
in the bat egg and this undergoes elimination, a process called 
deutoplasmolysis by the author. A similar condition is found 
in the ovum:of the armadillo by Newman (’12), but this author’s 
statement that the similarity in the distribution of deutoplasm 
in the eggs of the armadillo and of Dasyurus argues for the low 
phylogenetic position of the Edentata loses some of its force 
from the fact that the egg of Didelphys, a marsupial, does not 
exhibit a polar concentration of fat. 
1. Later cleavage to the formaiion of the blastocyst 
An extended description of the later cleavage of the opossum 
egg was presented in my former article (Hartman, 716), to 
which the reader is referred for details here omitted. The new 
material collected in 1916 and 1917 contains eggs from 8 to 26, 
28, and more cells; all corroborative of the former account. 
These eggs were also carefully studied in the living state and 
were photographed in salt solution at high and low magnifica- 
tions, and the assurance may be given that the fixed and 
sectioned material accurately represents the true morphological 
relations. This is well borne out by the photographic repro- 
ductions of living eggs and of sections made from them as shown 
on plates 4 and 5. 
JOURNAL OF MORPHOLOGY, VOL. 22, NO. l 
