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XIII. — The Formation, Rupture, and Closure of Ovarian Follicles in Ferrets and 
Ferret-Polecat Hybrids, and some Associated Phenomena. By Professor 
Arthur Robinson, University of Edinburgh. (With Ten Plates.) 
(Read July 9, 1917. MS. received April 26, 1918. Issued separately September 18, 1918.) 
Introduction. 
A few words of explanation are necessary at the outset in connection with 
the terms used in association with the animals upon which these investigations 
were made. 
Dealers in ferrets supply two types of animals — one with yellowish-white fur 
and pink eyes, which they call “ ferrets,” and a second type in which the yellowish 
fur is intermingled with a varying admixture of brownish or brown-black hairs ; 
the second type they call “ polecats.” The polecats of the ferret-dealers are not, 
however, true polecats, for Miss Frances Pitt, who has bred ferrets and polecats 
and ferret-polecat hybrids in a scientific manner, and to whom I am indebted for 
the greater and best part of my information on this subject, points out that the 
so-called polecat of the dealers is never so dark and handsome in colour as the 
true polecat. Moreover, as Miss Pitt states, in the true polecat the light marks 
in front of the ears do not usually join across the forehead, whereas in the polecat 
of the dealers they form a band across the face ; further, when viewed from in 
front and above, the head of the true polecat has the form of a fairly equilateral 
triangle, whilst in ferrets and the so-called polecats of the dealers the head, on 
the whole, has the form of an isosceles triangle. 
The polecat is classified as a sub-genus of the Mustelinse, and is termed 
M. putorius ; and the ordinary ferret is generally looked upon as a white variety 
of the same sub-genus. Doubt has been cast upon this assumption by Miller 
(26), who states that, on account of some cranial features, the common ferret is 
more closely allied with the Siberian M. eversmanni than it is with M. putorius. 
The features to which Miller draws attention cannot be denied, but Miss Pitt 
says that reddish specimens of the polecat are not uncommon in the Aberystwith 
district, and the red colouring is also met with in the common ferret ; this fact 
appears to point to a reversion of both forms to an ancestral type, though it in 
no sense substantiates the view that the ferret is a white variety of the polecat. 
Though the zoological position is of interest, it is not of great importance from 
the point of view of this investigation, for whatever the zoological affinities of 
the ferret and the polecat may be, it is certain, from the results of Miss Pitt’s 
observations, which are confirmed by my own experience, that ferrets and polecats 
breed freely together, and their progeny are fertile among themselves, with ferrets 
TRANS. ROY. SOC. EDIN., VOL. LII, PART II (NO. 13). 49 
