the uterus may, however, reasonably be supposed to give off something
which is absorbed into the maternal blood, and this something would be of
the same nature as that which was given off by the ovum while still within
the ovarian follicle. The presence of this hormone might cause the
follicular cells to behave as though the ovum was still present in the
follicle, so that they would persist and not die and be absorbed. But this
leaves the question, what is lutein and why is it secreted? Lutein is a
colouring matter sometimes found in blood-clots, and probably derived from
haemoglobin. In the corpus luteum the lutein is contained in the cells,
not in a blood-clot.

Chemical investigation shows that the lutein of the corpus luteum is
almost if not quite identical with the colouring matter of the yolk in
birds and reptiles. Escher [Footnote: _Ztschr. f. Physiol. Chem._, 83
(1912).] found that the lutein of the corpus luteum had the formula
C{40}H{56} and was apparently identical with the carotin of the carrot,
while the lutein of egg-yolk was C{40}H{56}O{2} and more soluble in
alcohol, less soluble in petroleum ether, than that of the corpus luteum.
The difference, if it exists, is very slight, and it is evident that one
compound could easily be converted into the other. Moreover, the
hypertrophied follicular cells which constitute the corpus luteum secrete
fat which is seen in them in globules. The similarity of their contents
therefore to yolk is very remarkable, and it may be suggested that the
hormones absorbed from the ovum or embryo in the uterus acts upon the
follicular cells in such a way as to cause them to secrete substances
which in the ancestor were passed on to the ovum and formed the yolk. It
may be urged that this idea is contradictory to the previous suggestion
that the absorption of nourishment by the intra-uterine embryo was the
cause of the gradual decline of the process of yolk-secretion by the ova
in the ovary, but it is not really so. Originally in the reptilian