CYCLIC CHANGES IN THE OVARY OF GUINEA PIG oo 
gressed and under such circumstances mitoses were no longer 
present in them. 
The corpora lutea of the preceding (II) generation, originating 
in an ovulation that took place at least thirty-seven days ago, 
are sometimes represented by small bodies which are surrounded 
by a thick connective tissue capsule; much fibrous tissue is found 
in the center and the lutein cells between these two zones show 
very large vacuoles. The vessels remaining in such structures 
have very thick cellular walls. In other cases some yellow pig- 
ment appears in such vacuolar cells and in still other cases we see 
only yellow.atretic bodies. It is probable that the latter struc- 
tures are found in cases in which a still longer time has elapsed 
since the preceding (second last) ovulation. There may of course 
have occurred a longer interval than twenty days between the 
last and second last ovulation. 
When the second generation was represented by a corpus luteum 
of pregnancy, the retrogressive changes were also marked, shrink- 
ing of the corpus luteum and vacuolization of the lutein cells are 
pronounced, but such corpora lutea are still considerably larger 
sixteen to nineteen days after the completion of pregnancy than 
ordinary corpora lutea of the corresponding generation. Some of 
the vacuolar cells may show a yellow pigmentation. In such 
ovaries we may find a still older generation of retrogressing cor- 
pora lutea present, represented by yellow atretic bodies which 
owe their origin to an ovulation that took place more than a 
hundred days ago; and if the last named (third last)ovulation were 
followed by a pregnancy, this ovulation may have taken place 
approximately one-hundred and fifty days ago. Not in all 
animals are so many generations of corpora lutea found; especially 
in young animals (two to three months old only one generation 
may be present. 
If the last ovulation that took place fifteen and one half to 
nineteen days ago were followed by pregnancy, the follicles in the 
ovaries of pregnant animals of this period do not show any marked 
difference from the follicles of non-pregnant animals at the corre- 
sponding period after ovulation. In both cases we find good folli- 
cles of various sizes and the different stages of retrogression of 
