above proposed, but it must be remembered that the ovum of Monotremes is
known to remain for a short period in the oviduct, or in other words to
pass through it very slowly, and to absorb fluid from its walls, as shown
by the considerable increase in size which the ovarian ovum undergoes
before it is laid. It would be interesting to know how long the
rudimentary corpus luteum persists in _Ornithorhynchus_: the period,
according to my views, should be very short. It is remarkable that in the
results quoted by Marshall a well-developed corpus luteum was found and
exclusively found in the lower Vertebrates which are viviparous. For
example, among fishes in the Elasmobranchs _Myliobatis_ and _Spinax_; in
Teleosteans, in _Zoarces_; in Reptiles, in _Anguis_ and _Seps_. Buehler on
the other hand, confirmed my own negative result with regard to oviparous
Teleosteans, and also found no hypertrophy of the follicle in Cyclostomes
which are also oviparous. In the viviparous forms mentioned there is yolk
in the ovum which is retained in oviduct or ovary, but additional
nutriment is also absorbed from the uterine or ovarian walls. In these
cases there is no placenta and generally no adhesion of ovum or embryo to
walls of oviduct or ovary. These facts alone would be sufficient to
disprove the theory that the corpora lutea are organs producing a
secretion whose function is to cause the attachment of the embryo to the
uterine mucosa. It is also, in my opinion, unreasonable to suppose that
the rudimentary corpora lutea of lower viviparous Vertebrates arose as a
mutation the result of which was to cause internal development of the
ovum. Habits might easily bring about retention of the fertilised ova for
gradually increasing periods, [Footnote: According to Geddes and Thomson
(_Evolution of Sex_, 1889), the common grass-snake has been induced under
artificial conditions to bring forth its young alive.] and the correlation
between the retained developing ova and the hypertrophy of the ruptured
follicles is comprehensible on my theory of the influence of substances
absorbed by the walls of oviduct or ovary from the developing ovum.