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PROFESSOR ARTHUR ROBIRSON ON 
remain quiescent for relatively long periods, in some animals for years, when, through- 
out the whole period, all are under the influence of exactly similar conditions. 
In the course of my investigations into the history of the follicles in the ferret 
I have noted a number of facts, which I hope to publish as soon as photographs of 
the specimens can be made, which are entirely opposed to the idea that the ova 
and follicles which become mature at different periods are all formed at one period 
of proliferation of the germinal epithelium ; and, although the work is not yet 
completed, I may state here that the results so far obtained not only confirm in 
all essential respects those arrived at by Kingery during his investigations on 
the development of ova in the white mouse, i.e. “ that the new formation of germ 
cells from the germinal epithelium is prolonged and extends from birth or shortly 
afterwards to approximately sexual maturity,” but that they lead me to the 
conclusion that new groups of ova and follicle cells are formed throughout the 
whole period of the functional life of the ovary. 
My observations do not extend over the period from the beginning of August 
to the end of October ; but during the remaining period of the year, that is, from 
the early part of November to the end of July, it may be said, speaking generally, 
that in ferrets and ferret-polecat hybrids the growth and death of groups of 
ovarian follicles proceeds continuously, and that it is not interfered with by 
pregnancy, but the statement, though it is true in its broad outlines, requires 
qualifications. 
In the first place-, if the corpora lutea are numerous and large they may occupy 
practically the whole of the cortex ; stroma, interstitial tissue, and follicles being 
reduced to a minimum. In these cases the follicles which are present, normal and 
degenerate, are either quite primitive or but slightly advanced beyond that condition, 
and it is probable that it is in cases of this kind that the animal has no second 
oestrus period during the breeding season ; this, however, is merely a supposition, and 
it is not supported by the material at my disposal, for five post-partum animals 
bearing upon the point of which I have definite notes give the following results : — 
F.D. X/16 (Table II). 11 days post-partum. Cast 7 young on 29th May. All 
young dead on 1st June. Killed 10th June, when showing slight signs of pro-oestrum. 
Corpora lutea corresponding with last pregnancy, 10 in one ovary and 6 in the 
other ovary. Both ovaries contained follicles of full pre-inseminal growth. 
P.D. A. 40/15 (Table IY). Cast 8 young on 8th May. In full heat 27th May. 
Killed 1st June. Corpora lutea corresponding with last pregnancy, 8 in one and 
4 in the other ovary. Both ovaries contained follicles of full pre-inseminal growth. 
F.D. Z 2 /16 (Table II), Cast 7 young on 6th May, and ate them all the same day. 
Was not in heat when killed on 2nd July. Corpora lutea corresponding with previous 
pregnancy, 8 in one and 4 in the other ovary. No follicles of full growth in 
either ovary. 
