THE OESTROUS CYCLE IN THE MOUSE Bot 
ovulation. In three mice of this series a distinct difference in 
the position of the ova in the tubes and a difference in degree of 
development of the corresponding corpora lutea may indicate 
that the synchronism is less perfect where mating or artificial 
insemination do not occur. In these three mice the ova ovulated 
at one oestrus were plainly separated into two groups with a 
day’s difference between their ovulation time. 
3. The number of ova produced at one ovulation. The num- 
ber of ova produced at an ovulation as an indication of the degree 
of fecundity of an animal is always of interest. When approached 
from the number in new-born litters, the failure of insemination, 
faulty implantation, intra-uterine death, and mortality at par- 
turition introduce an extensive error. Therefore, the problem 
is most accurately solved through ovarian studies. 
Counts of recent corpora lutea checked in many cases by com- 
parison with the number of corresponding ova in the tubes, the 
number of the next older set of red corpora lutea, and also the 
number of largest-sized follicles in prooestrous and oestrous 
ovaries give a total of 449 ova produced at forty-nine oestrous 
periods. This makes an average of nine ova to an oestrus. Of 
these 449, 230 were counted in left ovaries and 219 in right, show- 
ing the function almost equally divided between the two ovaries 
in virgin and non-mated animals. Long and Mark, in a large 
series, obtained an average of seven ova from the ovulation 
following parturition—a 22 per cent difference. It is possible 
that a supervening pregnancy reduces the number of ova pro- 
duced at the oestrus following parturition. 
4. The possibility of alternation of major function. The 
finding of a variation in the numbers of embryos in the horns of 
bicornuate uteri has raised the question as to alternation of 
major function between the two ovaries. In twenty-one ani- 
mals in which data were available for the number of ova pro- 
duced at two, three, and sometimes four oestrous periods, four 
show marked and two slight alternation of function, while in 
4 All large follicles need not rupture, which introduces a slight error into 
this data. 
