CYCLIC CHANGES IN THE OVARY OF GUINEA PIG 65 
cells decreases and they become more resistant to those processes 
which lead to degeneration in other follicles. The follicles in 
which such changes have taken place are mature and ready to 
rupture. In the meantime the follicles that ruptured during the 
preceding ovulation developed into corpora lutea. The latter 
represent principally the hypertrophic granulosa cells of the rup- 
tured follicles, which proliferate mitotically. After a certain 
stage of development has been reached, degenerative processes 
set in in the corpus luteum, which start in its periphery and pro- 
ceed to the center. These degenerative processes set in very early, 
are noticeable eighteen to twenty days and are usually marked 
twenty to twenty-four days after the preceding ovulation. 
Throughout this period of beginning degeneration, however, some 
mitoses are still visible in certain lutein cells. At this period 
usually a new ovulation takes place. The exact time at which 
the new ovulation occurs varies however somewhat in different 
animals, ovulation occurring earlier in some animals than in 
others. In some cases it can be hastened through certain external 
factors, especially copulation, but in the large majority of cases 
it occurs sooner or later even without a preceding copulation. 
After the new ovulation has taken place, the degenerative processes 
progress in the corpus luteum, although within the first twenty 
hours after ovulation mitoses may still be found in certain lutein 
cells. In the following period a considerable shrinking of the cor- 
pus luteum takes place; the connective tissue in the cortex and in 
the periphery becomes hyaline and forms a relatively prominent 
part enclosing a small number of very vacuolar cells. Gradually 
yellow pigment is deposited in these vacuolar cells and thus the 
corpora lutea become transformed into the atretic yellow bodies. 
The new ovulation was of course again followed by thetypical 
changes in the follicles. 
If the ovulation be followed by pregnancy, the principal changes 
taking place in the ovaries are on the whole the same. The only 
4 Whether or not in the guinea pig ovulation can take place independently of a 
preceding copulation has been a subject of controversy. Concerning the litera- 
ture of this question see William H. Kirkham, Biological Bulletin, ‘vol. 18, no. 5, 
April, 1910. 
