The case of _Dasyurus_, however, seems inconsistent with this argument,
for, as previously mentioned, Sandes found that in this Marsupial the
corpora lutea persisted during the greater part of the period of
lactation, which continues for four months after parturition. During the
whole of this time there are no embryos in the uteri, and therefore it
might be urged absorption of hormones from the embryos cannot be the cause
of the persistence of corpora lutea in pregnancy. But it seems to me that
a complete answer to this objection is supplied by the peculiar relations
of the embryos to the pouch in _Dasyurus_ and other Marsupials. The skin
of the pouch while the embryos are in it is very soft, congested, and
glandular; at the same time the embryos when transferred to the pouch at
parturition are very small, immature, and have a soft delicate skin. The
relation of embryos to pouch in _Dasyurus_, therefore, is closely similar
to that of embryos to uterus after the first few days of pregnancy in the
Eutheria. It is true there is no placenta, but the mouths of the embryos
are in very close contact with the teats, and both the skin of the embryos
and that of the pouch are soft and moist. If any special substances are
given off by the embryos in the uterus in ordinary gestation, the same
substances would continue to be given off by the embryos in the marsupial
pouch, and these must be absorbed by the skin of the pouch. In this way it
seems to me we have a logical explanation of the fact that the corpora
lutea in the Marsupial are not absorbed at parturition as in Eutheria. As
Sandes says the 'greater part of the period of lactation,' it would appear
that absorption of the corpora lutea takes place when the young _Dasyurus_
have grown to some size, become covered with hair, and are able to leave
the teats or even the pouch at will. Under these conditions it is obvious
that diffusion of chemical substances from the young through the walls of
the pouch would come to an end. It would be interesting in this connexion
to know more of the relation of egg and embryo to the pouch and to the