OVARIAN FOLLICLES IN FERRETS AND FERRET-POLECAT HYBRIDS. 335 
It is possible, however, that other factors may be at work, for I find that in the 
cases of females inseminated by the same male ovulation may occur in all, but 
fertilisation may fail in one or more. Thus if it is possible for the seminal fluid to 
produce ovulation in two females and fertilisation in only one of the two, it is also 
possible that the seminal fluid of a given male, independently of the number of 
spermatozoa, may be less potent in producing ovulation in one female than in another. 
The evidence at present available on this matter is quite insufficient to justify a 
positive statement, and the point must be left open. 
A question which naturally arises in association with the points which have 
already been noted is, is there any significant structural difference between the 
ovaries of the animals which ovulate spontaneously and the ovaries of animals which 
only ovulate after coitus ? It may be admitted at once that the evidence available 
does not justify a definite answer, but it suggests assumptions and indicates paths 
along which further investigations might be made, and on that account it deserves 
consideration. 
In the cat and ferret, which ovulate only after successful insemination, the 
interstitial tissue forms a preponderant part of the cortex of the ovary. In the 
human female, on the other hand, and in monkeys, mares, asses, pigs, and in 
Dasyurus viverrinus, all of which ovulate spontaneously, the interstitial tissue is 
either absent or it is present in only small amount, and not in the typical 
glandular form in which it occurs in the animals first mentioned ; but although 
there are two widely different types of ovaries so far as the presence of interstitial 
tissue is concerned, that tissue cannot be responsible for the different modes of 
ovulation, for it is present in the typical form and in large amount in guinea-pigs, 
which ovulate spontaneously, and in the rabbit, which sometimes, though rarely, 
ovulates spontaneously (25a), and in the bitch, which regularly ovulates spon- 
taneously ; for although Frankel denies its presence in that animal, it is present 
in considerable amount in some of my specimens. The bats give, at present, as little 
help in the solution of the question of the relation of ovarian structure to ovula- 
tion as they do to the question of the relation of follicular growth to follicular 
development. Their ovaries contain a very large amount of interstitial tissue, and 
we know that the majority, if not all, of the sexually mature females are inseminated 
in autumn and do not ovulate till the following spring ; but we also know that 
some females are inseminated in spring, and with regard to them there is no 
evidence as to whether they ovulate spontaneously or not. Moreover, it must not 
be forgotten that the arrangements in bats may be a case of special adaptation 
developed to assure the majority of the females being inseminated, in autumn, 
by vigorous, well-fed males rather than by males which have recently passed 
through a long period of hibernation. 
That such special adaptations do occur is evidenced by conditions met with in 
birds. In the case of the ordinary fowl the male takes no part in the incubation 
TRANS. ROY. SOC. EDIN., VOL. LII, PART II (NO. 13). 53 
