336 The Tortoiseshell Tomcat — A Suggestion 



features. There seems to be no reason for supposing that this is so, or 

 for expecting that it should be so, in a tortoiseshell cat of chromosome 

 constitution XO instead of XY (X- instead of X6). 



The purpose of this note is to put forward another suggestion — 

 equally speculative, but more easily capable of verification. In a recent 

 paper Magnusson (6) has described the anatomy, both gross and micro- 

 scopic, of some 70 free-martins. He finds that although in all of them 

 the external features are predominantly female, internally they approxi- 

 mate more or less nearly to the male. In more than half of them the 

 gonads were in the position of ovaries, the uterus was distinctly 

 developed and the vasa deferentia rudimentary, but even in these the 

 gonad was at least as much like a rudimentary testis in structure as like 

 an ovary. The examples described by Miss Chapin (1) from Lillie's 

 material seem to correspond with this group of Magnusson's cases. In 

 the remainder of his cases Magnusson found a series of stages in which 

 the uterus was more and more reduced down to complete absence, the 

 vasa deferentia and epididymis well developed, and in several instances the 

 gonads had passed into the inguinal canal. In these examples of more 

 male type the testis contained seminal tubules with interstitial tissue, 

 including Leydig's interstitial cells, between them. The tubules some- 

 times contained Sertoli cells, but never any trace of true seminal cells, 

 and Magnusson describes them as closely similar to those found in 

 retained (cryptorchid) testes of true males. Further, it should be noted 

 that in a number of his examples the interstitial tissue was much more 

 abundant than in normal testes. Magnusson's description of the micro- 

 scopic structure of the gonad in the more masculine of his free-martins 

 in Cattle immediately recalls the condition described by D. W. Cutler 

 and the writer (2) in the testis of a sterile tortoiseshell tomcat; the 

 structure of the cat's testis appears to be almost identical with that of 

 the gonad in some of the free-martins. This suggests the possibility 

 that the tortoiseshell tomcat may possibly be in fact a free-martin. 

 Lillie (4) has shown almost beyond doubt that the fi-ee-martin is derived 

 from a female embryo which has been "masculized " by the confluence of 

 its vascular system with that of a neighbouring male foetus. Magnusson, 

 not knowing Lillie's work, believes that the free-martin is one of a pair 

 of uniovular male twins, but his evidence for this is entirely uncon- 

 vincing, and it may be assumed with confidence that Lillie's explanation 

 is the true one. If in Cats, as in Cattle, the embryonic membranes of 

 two foetuses may coalesce so that their blood-system becomes confluent, 

 and if in them the same masculization of a female embryo results, 



