Somewhat similar processes take place in the ovaries of Teleostean fishes,
as I know from my own observations, but no corpora lutea are formed in
these, although the degenerating follicles in course of absorption
correspond to corpora lutea. The spawning of Fishes, usually annual,
corresponds to ovulation in Mammals, and in the ovary after spawning the
numerous collapsed follicles containing the follicular cells may be seen
in all stages of absorption. [Footnote: Cunningham, 'Ovaries of
Teleosteans.' _Quart. Journ. Mic. Sci._, vol. xl. pt. 1., 1897.] At other
times of the year sections of the ovary show here and there ova which
after developing to a certain stage die and undergo absorption with their
follicles.

In the higher Mammals (Eutheria) the corpora lutea show a special relation
in their development to the occurrence of pregnancy, that is to say, they
have a different history when ovulation is followed by pregnancy to that
which they have when the ova, from the escape of which they arise, are not
fertilised. When fertilisation occurs the corpus luteum increases in size
during the first part of the period of gestation (four months, or nearly a
half of the whole period in the human species). It then remains without
much change till parturition, after which it shrinks and is absorbed. When
pregnancy does not occur the corpus luteum is formed, but begins to
diminish within ten or twelve days in the human species and is then
gradually absorbed. According to O'Donoghue, in the Marsupial _Dasyurus_
there seems to be no difference either in the development of the milk
glands or of the corpora lutea between the pregnant and the non-pregnant
animal. Sandes [Footnote: _Proc. Lin. Soc._, New South Wales, 1903.]
showed that in the same species the corpora lutea persisted not only
during the whole of pregnancy, which Professor J. P. Hill [Footnote:
_Anat. Anz._, xviii., 1900.] estimates at a little over eight days, but