OVARIAN FOLLICLES IN FERRETS AND FERRET-POLECAT HYBRIDS. 331 
function, considered in the light of the evidence furnished by the ferret, may now 
be considered. 
The contention that it is a secretion produced by the ovarian follicles which is 
responsible for the phenomena of the pro-oestrum and the oestrus does not receive 
universal acceptance. 
Heape (ll), as the result of extensive observations on rabbits, arrived at the 
conclusion that “ it does not seem possible to accept the view that the stimulus 
which induces pro-oestrum and oestrus has its origin in the ovary,” and he believes 
that the phenomena are due to a substance of extraneous origin (“ gonadin ”), “ due 
to a change in the condition of the blood brought about by climatic influences 
and food.” 
Heape (9, 10, ll) has shown that in monkeys ovulation may occur without 
menstruation, and that menstruation may occur without, in some cases, a precedent, 
and in others any subsequent, ovulation. Leopold (l 7) has demonstrated the same 
facts in the case of the human female. It seems to. be mainly on these two groups 
of circumstances that Heape bases his belief that the ovary cannot possibly play a 
part in the production of the phenomena of heat ; but whilst he is convinced of 
the impossibility of looking upon the ovaries as causative agents in the production 
of the phenomena of the pro-oestrum and the oestrus, he makes no attempt to show 
how it ' is that the phenomena cease in sexually mature females after the ovaries 
are removed. 
Marshall (23) acknowledges the influence of the ovaries, but denies that the 
follicles are the responsible agents. He bases his objection to the follicles on the 
results of two experiments made on bitches, in which he pricked the protruding 
follicles of the ovaries, in one case from one to .three weeks before the expected 
period, and in the other two months before the period was due. In both cases 
oestrus appeared at about the expected time. The first animal was killed a week 
after the appearance of oestrus, and it was found that “ the ruptured follicles were 
very abnormal, since they contained large cavities surrounded by tissue resembling 
degenerate luteal tissue.” “The probable conclusion is that heat is not brought 
about by the ripening follicles, but that the process of maturation and the phenomena 
of the pro-oestrum and oestrus are both the results of some further factor which is 
probably to be sought for in the ovarian interstitial cells.” 
To this conclusion O’Donoghue (29) rightly objects that interstitial tissue is not 
present in many mammals which ovulate regularly ; and although- it may be doubted 
if interstitial tissue, or at all events cells which play the part of interstitial cells, 
are absent as frequently as is supposed, they are undoubtedly not present in many 
mammals in the typical glandular form met with in the rabbit, cat, and ferret. 
It is stated that they are absent in the bitch — that is, in the animal on which 
Marshall experimented — but in some bitch ovaries in my possession interstitial cells 
are undoubtedly present in considerable numbers in the form of strands of cells and 
