336 
PROFESSOR ARTHUR ROBINSON ON 
of the eggs, and ovulation occurs independently of the presence of the male. In 
pigeons, the male pigeon takes his fair share in the process of incu’bation. It would 
obviously be a disadvantage, therefore, if the female began to lay before pairing ; 
and Craig (6) has shown that if pairing is prevented, ovulation is delayed for a 
very considerable period. In this particular case the influence of the male appears 
to be purely of psychic character, and it is certainly unassociated with coition. 
The present knowledge which we possess of marsupials also helps very little 
towards the solution of the question at issue, for though O’Donoghue (29) has pointed 
out that interstitial tissue is absent from the ovaries of the polyprotodonts, and 
is present in the ovaries of the diprotodonts, and Hill and O’Donoghue (12) have 
found that in the polyprotodont Dasyurus viverrinus ovulation is spontaneous, 
we do not yet know whether it is spontaneous or not in other marsupials. 
There is still another structural difference between the ovaries of polyoestrus 
spontaneously ovulating animals and the ovaries of the animals which only ovulate 
after copulation, that is, the presence in the former group of periodic corpora lutea, 
which develop after ovulations which are not followed by pregnancy. Periodic 
corpora lutea are structurally similar to the corpora lutea of gestation, but they 
persist for a much shorter time. In 1909 Bogin and Ancel (5) formulated the 
opinion that the interstitial gland is not present in spontaneously ovulating animals, 
and that its place is taken by the periodic corpora lutea, both structures presiding 
over the development of the secondary female sexual characters. The opinion 
cannot be sustained, since the base on which it rests is unsound ; for the guinea- 
pig, which ovulates spontaneously, and therefore can form periodic corpora lutea, 
also possesses a very large amount of interstitial gland tissue, and many other 
spontaneously ovulating animals have a considerable amount of interstitial tissue in 
their ovaries. 
So far as the evidence available goes, it seems to exclude interstitial tissue 
as an active agent in the control of ovulation, and we are driven towards the 
tentative conclusion that in animals that ovulate spontaneously something is 
present or something is absent as contrasted with animals which ovulate only 
after coition. 
If something is present in the first group which favours spontaneous ovulation, 
it must be absent in the animals which ovulate only after coition, and must be 
provided by coition ; and if something is present in animals which ovulate only 
after coition, which interferes with ovulation, it must be neutralised by coition, and 
it must be absent in animals which ovulate spontaneously. If either inference 
is correct, the question arises, How is the substance provided by coition, and is 
the end attained by chemical means, or by psychic means, as in the case of 
pigeons ? 
The evidence afforded by ferrets is opposed to the idea of psychic influence, for, 
in their case, sterile coition is not followed by ovulation, and it is difficult to believe 
