OVARIAN FOLLICLES IN FERRETS AND FERRET-POLECAT HYBRIDS. 333 
at the end of six weeks, when the animal was killed (21). In one of my specimens it 
had lasted thirty-two days when the animal had to be killed, in the summer of 1914, 
and I have not since then had an opportunity to carry the investigation further. 
It is, therefore, uncertain how long heat may continue in the case of the ferret. The 
time is obviously variable, and is no doubt associated with the fact that some ferrets 
produce one brood, others two, and a few three in the sexual season ; which prob- 
ably means that the substance necessary for the maturation is present in the 
first case only in sufficient amount to bring one group of follicles to pre-inseminal 
maturation, and is then so used up that there is not time, during the succeeding 
forty-two days of pregnancy, for the necessary recuperation to an extent sufficient 
for the production of the maturation of another group of follicles during the season ; 
or that the groups of follicles which grow and die during the gestation are not 
sufficient to produce the necessary amount of material, whilst in the second and 
third group of cases the conditions are more, favourable. 
Marshall gives no account of the ovaries of his two ferret specimens referred to 
which were kept separate from the male. In my specimen, which was killed in the 
thirty-second day of oestrus, the ovaries contained fourteen large follicles, of which 
five were in a state of degeneration ; the remainder were in the stage of development 
always found associated with the condition of heat (fig. 39, PI. VII). 
In ferrets, therefore, oestrus only occurs when the ovaries contain follicles in a 
certain stage of development, and such follicles are present as long as oestrus con- 
tinues (see Tables III, IV, V). Under these circumstances it can scarcely be doubted 
that the phenomena of heat are due to something produced by the follicles, and 
Marshall’s experiments prove not that the follicles take no part in the production 
of the phenomena of the pro-oestrum and the oestrus, but that puncture of follicles of 
considerable size, which have not yet attained the proper stage, does not prevent 
the follicular epithelium reaching the necessary stage and performing the functions 
associated with it. 
This part of the subject cannot be dismissed without some reference to the events 
which occur in the bats, for they have been used as an argument against the possi- 
bility that the follicles can play any part in the production of the phenomena of heat. 
It is known from the researches of Van Beneden (38) and Van der Stricht (39) that 
the majority if not all the sexually mature females are inseminated in autumn, and 
that the spermatozoa lie dormant till the following spring ; then some of the ovarian 
follicles ripen, ovulation occurs, and fertilisation follows. This evidence, so far as it 
goes, tells against the position I have taken ; but, unfortunately, we have no clear 
knowledge of the condition of the ovaries of bats in autumn, when it must be 
assumed that heat occurs, and until that is furnished, the evidence offered by bats 
has no great weight either one way or the other. There are also other possible 
explanations of the phenomena which occur in' bats to which it will be necessary to 
refer in connection with the next section of this discussion ; but before passing to 
