THE OESTROUS CYCLE IN THE MOUSE 345 
6. Ovulation is not always spontaneous in virgin or unmated 
mice. In some it is regularly spontaneous, in others sporadi- 
cally so, and in a few it seldom occurs without an added stimulus. 
7. The average duration of the cycle is four to six days. The 
mode of the curve for brown mice is six days; for yellows and 
erays, five days; for blacks and albinos, four days. The yellows 
and grays were descendants from one pair of mice, the albinos 
are browns minus the color factor. Therefore, a genetic fac- 
tor seems partly responsible for variation in cycle duration, and 
this may be tied up with the determiner for coat color. 
8. Ovulation need not necessarily be synchronous in unmated 
mice. 
9. Pregnancy may reduce the number of ova produced at 
the ovulation following parturition. 
10. There is no difference discernible histologically between 
corpora lutea of oestrus and pregnancy during the first four 
days of their development. 
11. Corpora lutea of oestrus do not normally inhibit ovula- 
tion in the mouse. 
12. In mice that ovulate spontaneously two or three sets of 
corpora lutea of oestrus may be present at all times; in mice that 
do not ovulate spontaneously corpora lutea may be entirely 
absent, and yet normal oestrous cycles are experienced in both 
types of animals. Therefore, the writer concludes that they 
have no primary causative relation to oestrous changes in the 
genital tract. 
13. As ovulation or the beginning of atresia of follicles is the 
dividing line between the anabolic and catabolic phases, and as 
the genetic factor summarized in no. 8 points to the ova them- 
selves, the conclusion is drawn that the presence of maturing 
ova in large follicles is the cause of the prooestrum and oestrus, 
and that the removal of the ova at ovulation (or their atresia 
if this fails to occur) is the primary cause of the degenerative 
changes of the metoestrum. 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 30, NO. 3 
