result from the implantation of ovaries from other individuals in the
condition of oestrus.

Before considering further the question of the corpora lutea as organs of
internal secretion, we may briefly refer to the origin and structure of
these bodies and of other parts of the mammalian ovary. The mature
follicle containing the ovum differs from that of other Vertebrates in the
fact that it is not completely filled by the ovum and the follicular cells
surrounding it, but there is a cell-free space of large size into which
the ovum covered by follicular cells projects. In the wall of the follicle
two layers are distinguished, the theca externa, which is more fibrous,
and the theca interna, which is more cellular. In the connective tissue
stroma of the ovary between the follicles are scattered, or in some cases
aggregated, epithelioid cells known as the interstitial cells, and it is
stated that the cells of the theca interna are exactly similar to the
interstitial cells. According to Limon [Footnote: _Arch. d'Anat. micr._,
v., 1902.] and Wallart [Footnote: _Arch. f. Gynock_, vi. 271.] the
interstitial cells are actually derived from those of the theca interna of
the follicles. Numbers of ova die without reaching maturity, the
follicular cells degenerate, and the follicle becomes filled with the
cells of the theca interna, which have a resemblance to those of the true
corpus luteum. These degenerate follicles have been termed spurious
corpora lutea, or atretic vesicles. The interstitial cells are the remains
of these atretic vesicles. The true corpora lutea arise from follicles in
which the ova have become mature and from which they have escaped through
the surface of the ovary. As a result of the escape of the ovum and the
contents of the cell-free space, the follicle contracts and the follicular
(so-called granulosa) cells secrete a yellow substance, lutein, and
enlarge. Buds from the theca interna invade the follicle and form the
connective tissue of the corpus luteum.