THE OESTROUS CYCLE IN THE MOUSE 299 
of ovulation after the interference of pregnancy. The variation 
in this period is known to be great even within a single species, 
therefore the conclusions drawn can scarcely be applied to the 
normal ovarian cycle. 
Estimates of previous oestrous periods arrived at from histo- 
logical comparisons of the corpora lutea are not reliable since, 
as will be shown in the present paper, many mice when isolated 
from males do not ovulate spontaneously during oestrus. 
Daniels (10) and King (13) have shown, in the mouse and 
rat, respectively, that if an animal becomes pregnant at the 
oestrous period following parturition and suckles her litter as 
well, the gestation period of her second litter may be lengthened, 
in some cases to from twenty-four to thirty days, an increase of 
20 to 50 per cent. Kirkham (’16—’17) showed this delay to be 
due to a failure of the embryos to implant in the uterine mucosa. 
The embryos apparently remain free in the uterus in a state of 
inhibited growth for a time equal to the extension of the gesta- 
tion period. No histological differences have been reported 
between the uterine mucosa of pregnant mice and those preg- 
nant and also lactating. Long and Evans (’20), working on the 
rat, have reported histological difference between corpora lutea 
under these two conditions and (’21) a limiting influence of com- 
bined pregnancy and lactation on the growth of the vaginal 
epithelium. For several reasons, therefore, it seems better to 
time embryological material from a mating during a heat period 
not immediately preceded by parturition. 
Further literature on the oestrous cycle in the mouse is limited 
to isolated observations made incidentally during investigations 
of other problems. 
Concerning the literature on other rodents Marshall (’10) ac- 
cepts Heape’s statement that ‘‘dioestrous cycles recur for five or 
six months in the domestic rabbit, and that if oestrus is ex- 
perienced in winter it may occur independently of the possibility 
of pregnancy. While some animals exhibit oestrus every three 
weeks fairly regularly, others do so every ten days; on the whole, 
I think 10-15 days is the usual length of their dioestrous cycle.” 
