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THE OESTROUS CYCLE IN THE MOUSE 325 
Ova may be found in the tubes during the early D interval, 
and if this is very short, may still be present in the succeeding 
P stage. They remain in good condition in the oviduct, with 
the second maturation spindles and sometimes the polar bodies 
intact, for two and even three days. During the third day, in 
the segments proximal to the cornua, they may begin to frag- 
ment. Their degeneration here must be by autolysis, as no 
leucocytes are present in the lumen of the oviducts. If occa- 
sionally they remain intact and pass into the uterine cornua on 
the fourth day after ovulation, phagocytosis may be their fate, 
for leucocytes are present there if the mouse has not passed the 
dioestrous interval. 
6G. The ovaries 
In a consideration of the ovarian cycle, two main subdivisions 
will be made for animals that do and those that do not ovulate 
spontaneously (i.e., without the added stimulus of sexual con- 
tact). Those only occasionally ovulating spontaneously are 
obviously intergrades and will not be considered separately. 
Where ovulation is spontaneous, three ovarian structures 
are involved: the follicles, the corpora lutea forming after their 
rupture, and possibly interstitial tissue. Where ovulation is 
not spontaneous there are, of course, no corpora lutea present, 
but atretic follicles take on an added significance. 
Large, normal follicles are always present in the P and O stages, 
while none are to be found in the M, period. This, with the 
added evidence deduced from the position of the ova in the tubes, 
checked by standardized corpora lutea, shows ovulation to 
occur at the end of oestrus. The follicles usually rupture syn- 
chronously, but this is not necessarily so. In twenty-seven 
animals studied histologically three showed almost a day’s 
difference between the position of the ova in the tubes, which is 
checked by a difference in the degree of development of the cor- 
responding corpora lutea. It takes a period equal to at least 
one oestrous cycle for the maturation of the follicle from a medium 
sized stage with primary liquor folliculi forming at the poles to a 
large one greatly distended with a single lake of liquor (fig. 20). 
