40 CARL G. HARTMAN 
large blastomere has two nuclei (text fig. 4, K, L, M) and in 
one egg the cytoplasmic division is initiated, as indicated by a 
constriction around the cell. The interesting point in these 
eggs lies in the position of the blastomeres to one another, 
especially in eggs Nos. 306 (3) and 173 (8); for the lines joining 
sister nuclei are almost absolutely at right angles to each other. 
The usual position of the blastomeres of the 4-celled stage 
(text fig. 4 N to P; figures on pl. 15) is, therefore, already 
anticipated in the 3-celled egg. The shifting of the blastomeres 
in egg No. 203 (3); shown in text figure 4, M, is rather along 
the original plane of the 2-celled stage, and such an egg might 
develop a 4-celled egg like that shown at O, whereas an egg like 
No. 173 (8), shown at L, would be sure to develop into the 
typical ovum with cross-shaped blastomeres, as in figures 5 
and 6, plate 15. 
2. The 4-celled stage. If the number of specimens which the 
collector happens to secure of a given stage be any criterion of 
the relative length of time which the egg remains in that stage, 
then according to my collection the 4-celled condition of the 
opossum egg is not passed very quickly. For I have more than 
five dozen, mostly excellent preparations of this stage, and have 
other eggs still unsectioned. Three whole litters (Nos. 293, 299, 
320) furnished only 4-celled eggs so far as these have been 
studied. However, this preponderance of 4-celled eggs is prob- 
ably quite accidental. Inasmuch as cleavage proceeds irregu- 
larly after the 4-celled stage, it would not be fair to compare the 
number of 4-celled with the number of 8-celled eggs, for ex- 
ample, for a litter preponderatingly 8-celled would be sure to 
contain 6-, 7-, and perhaps 10- and 12-celled eggs also (compare 
Nos. 85, 117, 342). 
The 4-celled egg of the opossum is typically Eutherian in the 
cross-shaped arrangement of the blastomeres. This is quite 
evident from the figures presented, some of which are drawn 
from models, others from in toto preparations and from sections 
(text fig. 4 and pl. 15). The arrangement: of the blastomeres is 
such that no section can possibly pass through the centers of all 
the four blastomeres. If the imbedded egg be so oriented that 
