CYCLIC CHANGES IN THE OVARY OF GUINEA PIG 67 
in in the corpora lutea, either during pregnancy or outside of 
pregnancy, a new ovulation can take place. How far the presence 
of the corpora lutea influences the transformation of ordinary large 
follicles into mature follicles and how far its action merely con- 
cerns the rupture of the mature follicles, remains still to be deter- 
mined. 
It follows from our observations that the time of ovulation 
depends upon at least three different factors: (1) Changes taking 
place in the ovaries. It is necessary that mature follicles have 
been produced, before rupture can take place. Our experiments 
indicate that cuts into an ovary causing an opening of a 
follicle may possibly lead to the formation of a corpus luteum only 
at a time when mature follicles are present. A certain time must 
therefore have elapsed after ovulation before another ovulation 
ean take place. During this period small follicles reach their 
full size. Thus a minimal time which must elapse between two 
ovulations is required. (2) The time at which the influence of the 
corpus luteum preventing ovulation ceases to be exerted. Our 
observations make it very probable that the retrogressive changes 
observed in the corpora lutea before ovulation indicate the neces- 
sary cessation of functional activity. It is however noteworthy 
that, notwithstanding such a cessation of activity, mitoses can 
still be observed in the lutein cells at this period. Whether the 
corpus luteum acts prineipally upon the last stage in the develop- 
ment of follicles (maturation) or merely upon the rupture of folli- 
cles will still have to be determined with certainly. We recall . 
however the fact that we observed the occurrence of mature 
follicles during various stages of pregnancy, notwithstanding the 
existence of corpora lutea. (3) Certain more or less accidental con- 
ditions, as for instance copulation. It is probable that other cir- 
cumstances also may accelerate or retard the rupture of the follicles. 
Such factors act probably indirectly by causing changes in the 
circulation in the ovaries. In the guinea pig these are not indis- © 
pensable, but theirplace can be taken by other factors; or even the 
total absence of corpora lutea may in the guinea pig be sufficient 
to allow a new ovulation. 
