ACTION OF SEX HORMONES IN FOETAL LIFE 373 



from time to time, that it applies to human twins, but this 

 is certainly not the case. In cattle, in about 87 per cent of ap- 

 parently different-sexed t^\dns, as nearly as I can ascertain, the 

 female is sterile; about 13 per cent are normally fertile (see data 

 beyond p. 381). We must therefore distinguish sterile and fer- 

 tile free-martins, and it is important to note that the fertile in- 

 dividuals are not known to be in any respect inferior to other 

 females in respect of breeding. The sterile individuals have the 

 external organs of a female, usually, but the internal organs of 

 reproduction are more or less of the male type. Their general 

 bodily appearance is more or less intermediate between a male 

 and a female — ^it has been compared to that of an ox or spayed 

 heifer — so that an experienced cattle man can usually distin- 

 guish them from normal heifers. The bull twin is always, nor- 

 mally fertile, and does not exhibit any anatomical pecuharities 

 so far as is known. 



It is essential to recognize the fact that the sterile free-mar- 

 tin condition is found only in association with a bull twin.^ The 



3 Numan ('43) is, so far as I know, the only author who has questioned this; 

 his study is by far the most extensive, and in many respects the most thorough, 

 that has been made on the free-martin. The publication is exceedingly rare, but 

 I have been able to study a copy from the library of the Smithsonian Institution, 

 and as his conclusions are so often quoted from author to author, it seems worth 

 while to give the evidence on which his dissenting statements are based. (D. 

 Berry Hart has published an abstract of Numan' s paper (Hart '12). Numan 

 states "The anomaly occurs not only in twins of different sex, but also in female 

 and male pairs, though more rarely." In these cases he refers first to an indi- 

 vidual about two years old, judged by external signs alone to be a sterile free- 

 martin; the owner stated that it was born twin to another female, which how- 

 ever was sold shortly after birth about two years previously. This would appear 

 to be slender evidence upon which to base a unique exception, for it was not 

 positively certain that the individual was a sterile free-martin, nor could Numan 

 know unquestionably that it was born twin to another female. It is known also 

 that cystic degeneration of the ovaries may lead to extensive assumption of 

 male secondary sex characters in the cow (Pearl and Surface, '15); the case may 

 belong in this category. This was the only case he had on the female side, and 

 no others have been recorded since. In the case of male pairs he cites also a 

 single case which is of great interest, but wrongly interpreted by him. It was 

 a case of twins one being a normal male and the other a sexually abnormal indi- 

 vidual. He judged the abnormal individual to be male on account of the pres- 

 ence of testes in the groins, and malformation of the external parts. The scro- 

 tum was absent. This is almost certainly an extreme case of modification of the 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 23, NO. 2 



