96 CARL G. HARTMAN 
later cleavage show an accelerated rate of division at one pole. 
The evidence of polar differentiation in the opossum egg through- 
out cleavage seems, therefore, to be complete. Professor Hill 
recognizes fully the difference between the cleavage of the 
opossum and of Dasyurus and joins me in deriving the formative 
and the non-formative areas each from one of the two blastomeres 
of the 2-celled egg. 
As to the method of deutoplasmolysis, Hill considers that‘‘ the 
yolk spheres are budded off from a narrow, clear zone which 
has made its appearance at the exposed surfaces of the blasto- 
meres” and his ‘‘figures shown undoubted yolk spheres in direct 
continuity with the lighter peripheral zone.’”’ I have noted the 
same phenomenon, although never as pronounced as in the cases 
illustrated by Hill in his figures 11 to 138, plate 8. In most of 
my hundred specimens, the smooth, unwrinkled cell membrane 
can be followed clearly around the blastomeres. Professor Hill 
is correct in assuming that the egg No. 50 (6) upon which I 
based my former conclusion on the method of yolk elimination 
(Hartman, 716, page 23, and fig. 9, pl. 5) is probably not quite 
normal; in fact, the specimen was considerably retarded in 
development as compared with its fellows in the 50 to 70-celled 
stage. Such retarded eggs are always to be regarded with 
suspicion. I therefore no longer regard yolk elimination as due 
to the “formation of a new cell membrane, . . . . ata 
distance from the original surface of the blastomeres,” but 
believe with Professor Hill that masses of variable size are 
extruded from different places on the exposed surfaces of the 
blastomeres. 
On the origin of the crossed arrangement of the blastomeres 
in the 4-celled egg, Hill presents evidence, which taken by itself, 
would be conclusive of the fact that the blastomeres do not 
attain this position by shifting, but assume it from the beginning 
by virtue of a meridional division of one blastomere and an 
~ equatorial division of the other. For out of his ten 2-celled eggs, 
five have both blastomeres in the process of division and in these 
the axes of the blastomeres are already nearly or quite at right 
angles to each other. Three of these eggs have completed their 
