ancestor, or in the Monotreme, the ovum in the follicle secreted
yellow-coloured yolk. The materials for this, at any rate, passed through
the follicle cells, and it is probable that these cells were not entirely
passive, but actively secretory in the process. Substances diffusing from
the ovum would be present in the follicle cells during this process, and
probably act as a stimulus. The same substances diffusing from the ovum
during its development in the uterus would continue to stimulate the
follicle cells, and thus explain not merely their persistence, but their
secretory activity. The ovum being no longer present in the ovary, the
secretions would remain in the follicular cells, and the corpus luteum
would be explained.

If this theory is sound, it would follow that corpora lutea are not formed
in cases where the ova are not retained in the oviduct during their
development. The essential process in the development of these structures
is the hypertrophy and, in some cases at least, multiplication of the
follicular cells in the ruptured follicle. I have already mentioned that
this process does not occur in Teleosteans whose ovaries were studied by
me. These were species of Teleosteans in which fertilisation is external.
Marshall, in his _Physiology of Reproduction_, [Footnote: London, 1910, p.
151.] quotes a number of authors who have published observations on the
changes occurring in the ruptured follicle in the lower Vertebrata, and
also in the Monotremes. According to Sandes, [Footnote: 'The Corpus Luteum
of Dasyurus,' _Proc. Lin. Soc._, New South Wales, 1903.] in the latter
there is a pronounced hypertrophy of the follicular epithelium after
ovulation, but no ingrowth of connective tissue or blood-vessels from the
follicular wall. Marshall himself examined sections of the corpus luteum
of _Ornithorhynchus_ and saw much hypertrophied and apparently fully
developed luteal cells, but no trace of any ingrowth from the wall of the
follicle. This fact would appear to be quite inconsistent with the theory