DEVELOPMENT OF THE OPOSSUM 29 
rate is not affected by the abdominal operations such as em- 
ployed in our experiments. 
TABLE 4 
Number of abnormal embryos of later stages 
= NUMBER OF TOTAL NUMBER 
LITTER NUMBER INTERVAL lea ae peau cee eet ies 
207 0 11 1 i 
207’ 2? days 5) 7 10 
226 (left) 0 3 4 7 
226 (right) 0 0 all 
291 0 10 0 10 
291’ + day 7 0 if 
379 0 2 4 6 
379’ 18 hours 6 0 6 
314’ 53 days 6 0 6 
336/ 5 days 6 2 8 
342’ 7 days 9 2 11 
349’ 33 days 9 1 10 
360’ 3 days 18 2 20 
MATURATION AND CLEAVAGE TO THE FORMATION OF THE 
BLASTOCYST 
a. The ripe ovarian egg 
Since the publication of my former paper I have not seen the 
first maturation spindle, for the ova of all large follicles thus 
far studied in numerous series of ovaries have either germinal 
vesicles in the resting stage just preceding maturation or have 
already given off the first polar body. Recently collected 
ovaries containing large follicles are now being prepared and 
will be discussed in connection with a paper on the corpus 
luteum. 
The ripe ovarian egg is broadly elliptical in form, but may 
be nearly spherical, as some dissected specimens indicate. 
Measurements were previously stated to average 0.165 x 0.135 
mm. or larger than any tubal or uterine ova. This is large in 
comparison with Futherian ova, but small in comparison with 
