THE OESTROUS CYCLE IN THE MOUSE O27 
more distended and an increase in size is apparent in the cor- 
pora lutea. 
In the second class of animals (those which do not ovulate 
spontaneously) the follicles attain their largest size during oes- 
trus, which may be considerably prolonged. The follicles failing 
to rupture, the cumuli disintegrate and atresia begins in the 
granulosa cells, continuing until the follicular epithelium is en- 
tirely gone. Contained ova may fragment in the late stages of 
follicular atresia or they may persist with maturation spindles 
intact until most of the granulosa cells have disappeared. The 
finding of several distinct sets of abnormal follicles in progressively 
later stages of atresia when no corpora lutea are present in the 
ovaries, but several oestrous periods have been recorded for the 
animal, makes it possible to approximately estimate the time re- 
quired for a certain degree of follicular atresia. In late stages 
of degeneration, former large follicles are reduced to medium 
and even small size. Record of previous oestrous cycles is neces- 
sary to properly evaluate the conditions of follicular atresia in 
the ovary of the mouse. 
Interstitial tissue is present in the ovaries in varying but 
usually small amounts in the majority of the animals in this 
series. Its distribution is for the most part peripheral. Its 
presence or amount is not peculiar to any stage of the cycle. 
7. TIME RELATIONS OF THE CYCLE 
The duration of the cycle and of its various stages shows 
great variability. This is represented in the curve (graph 1) 
obtained by plotting the number of cycles against their duration 
in days. A total of 563 cycles is included. The mode of the 
curve falls at 45 days. An average duration of four to six days 
therefore represents the findings. 
The dioestrous interval shows a greater variation than any 
other stage, lasting from less than a day to as long as fourteen 
days. Its length is usually from one to three days, however. 
In several cases of an extremely long D interval, the smears may 
at times show considerable amounts of cornified cells but the pres- 
ence of leucocytes makes evident the diagnosis of the stage D. 
