DEVELOPMENT OF THE OPOSSUM Y9) 
have seen them in numerous eggs in cleavage, especially in 
4-celled eggs. The oldest stage which contains two objects 
that I take to be polar bodies is a blastocyst of 34 cells. The 
polar bodies are caught between two blastomeres of the vesicle 
(fig. 1, pl. 16). The larger of the two is shaped like a bent 
spindle and resembles in outline the space which it occupied 
while still crowded in the usual periovarial space before cleavage. 
I have never seen polar bodies so large that they appear in 
cross-section like those figured by Spurgeon and Brooks (’16). 
5. The chromosomes. The spindle for the second maturation 
division is formed soon after the giving off of the first polar body, 
and in this condition the egg reaches the Fallopian tube. The 
vesicular or resting stage does not seem to intervene between 
the two maturation processes. Insemination was not observed. 
In the absence of spermatozoa, the ovum reaches the uterus 
unchanged, except for the accession of the egg envelopes. Thus 
the young uterine eggs Nos. 58, 287, and others have chromo- 
somes practically indistinguishable from those about to be 
described for tubal ova. 
Three preparations from batch No. 307 (fig. 7, pl. 1; figures 
on pl. 14) are especially favorable for a study of the chromosomes 
and for determining their number. These are still scattered 
along the clearly defined spindles, the equatorial plate being 
delayed in its formation. Some of the spindle fibers are thick 
and beaded as though they were derived from the fibers of the 
preceding division. One spindle is contained in a single section 
(fig. 6, pl. 14). There are clearly twelve chromosomes in each 
of these eggs. In all other tubal ova the chromosomes are 
closely arranged in a more or less definite equatorial plate and 
are difficult to count; but I am sure that the number is twelve 
in five or six cases, and I can count at least ten or eleven chromo- 
somes with distinctness in all cases. Hence I am prepared to 
state that twelve is the reduced number of chromosomes in the 
egg of the opossum. 
Figures 11 and 14, plate 14, represent the usual appearance 
of the chromosomes in these specimens, and in these two cases 
twelve chromosomes can be clearly made out. Figure 13 shows 
