STERILITY 129 



and those which are immune. The great value of this 

 lies in the fact that the aborting cows may be entirely 

 separated from the healthy members and eventually 

 by this separation the disease may be entirely eliminated 

 from the herd. 



122. Sterility of free-martins. The birth of twins 

 among cattle is frequent. When a cow gives birth to 

 twins, one a female and the other a male, the female is 

 called a free-martin and is generally sterile. So far as 

 known, this condition does not exist among any other 

 known species of animal. Among sheep, for example, 

 where twins are very common, the female twin born with 

 a male may be even more fertile than the single born 

 lamb. No case of sterility among human twins has ever 

 been recorded where the sterile condition was believed 

 to be due to the fact that one twin was a male and the 

 other a female. Among cattle where both twins are of 

 the same sex both are fully fertile. This is, therefore, 

 a remarkable biological fact which it is difficult to explain. 

 Morse 1 reports that Dr. Luer found 113 cases of twins, 

 one a male and the other a female, in the records of the 

 East Prussian Holland Herd Book. Of this number all 

 the females were sterile except six. 



The author has examined a considerable number of 

 free-martins and in every case of sterility the female 

 reproductive organs have been imperfectly developed. 

 One case examined is typical. A grade Aberdeen Angus 

 had every external appearance of a female. The loca- 

 tion and form of the external genital organs was that of 

 a true female. There were only two peculiarities which 

 were visible externally. A tuft of long hair resembling 

 the growth on the sheath of the bull grew from the lower 



1 Morse, Breeder's Gazette, vol. 64, p. 346. 



