CYCLIC CHANGES IN THE OVARY OF GUINEA PIG 47 
the corpus luteum we find a relatively small amount of fibrous 
tissue. We not only find capillary vessels but also vessels the wall 
of which consists of two coats penetrating the corpus luteum. 
These corpora lutea begin to shrink very soon, and three days 
after the new rupture they are usually smaller than immediately 
after the ovulation. Notwithstanding the degenerative processes 
which are apparent in these corpora lutea, it is not uncommon to 
to find still mitoses in the lutein cells of such corpora lutea within 
the first twenty hours after the new rupture of follicles has taken 
place. At a later period mitoses were not seen in this series. The 
mitoses appear in the relatively well preserved, but they may be 
present even in somewhat vacuolar lutein cells. It is possible 
that occasionally mitoses occur also in endothelial cells of the capil- 
laries. © 
3. The third generation is represented by yellow hyaline bodies. 
They are the remnants of corpora lutea that formed forty or more 
days ago. 
If we examine ovaries of young guinea pigs, two and a half to 
three months old, we may find only the first, or the first and second 
generations of corpora lutea, but yellow bodies may be lacking. 
We see therefore that preceding and following the rupture of 
new follicles in non-pregnant animals, processes of degeneration 
have begun in the corpora lutea of the preceding ovulation, and that 
notwithstanding such processes of degeneration, mitoses may 
occur in such corpora lutea for a short period following the new 
ovulation. These corpora lutea which are not accompanied by 
pregnancy are much smaller than the corpora lutea of pregnancy 
and they shrink more rapidly. The absolute diminution in size 
is more rapid than in the retrogressing corpora lutea of a preced- 
ing pregnancy. Concerning the relative rapidity of retrogression 
(the percentage decrease in size, the full size of the corpora lutea 
being taken as the standard), we cannot make any definite state- 
ment, not having carried out any measurements. 
The mode of retrogression is the same in both ordinary corpora 
lutea and in those of pregnancy. The vacuolization begins in 
the periphery, where it becomes most marked, and from here it 
proceeds into the interior of the corpus luteum. 
