HEREDITY AND SEX 131 



The Free-Martin. — When cows have twins it sometimes 

 happens that one calf is a normal fertile bull and the other a 

 sterile individual, A\hich is generally regarded as an abnormal 

 female with some of the characters of a bull. This is called a 

 free-martin. There is considerable variation among free-martins, 

 but in the more typical examples an enlarged clitoris is present, 

 and internally vesiculoe seminales and vasa deferentia are generally 

 represented, though in an undeveloped state, together with 

 a rudimentary uterus. The gonads are often testis-like. but 

 they contain no spermatogenetic tissue, and are retained in 

 the body cavity ; sometimes they appear to be undeveloped 

 ovaries. 



It was at one time suggested that the free-martin, together with 

 its co-twin, might arise from a single ovum. That two embryos may 

 originate from the segmentation of one ovum is known occasionally 

 to occur, but in such cases the two individuals usually bear a 

 very close likeness to one another, so much so that they have 

 been compared with looking-glass images, and they are described 

 as " identical twins." The fact that the free-martin and 

 its partner are not derived from the same egg is, however, 

 definitely negatived by there being two corpora lutea in the 

 ovaries of the mother, thus showing that two follicles had ruptured 

 and two ova had been discharged. 



The probable explanation of the free-martin was discovered 

 by Lillie in America, and independently by Keller and Tandler 

 in i\.ustria. They found that the chorionic membranes of the 

 two embryos — the free-martin and its twin male — were fused 

 together, and not only this but the blood vessels were also con- 

 nected together so that there was a common circulation between 

 them. This suggested that substances formed in one of the 

 embryos would pass to the other and modify its development. 

 In view of the other known facts about hormonic action in 

 modifying sex it was natural to suppose that the free-martin 

 started its life as a female, but was acted on by the male hormone 

 coming from the bull twin. Moreover, it was shown that when 

 two embryos are present without any connection between the 

 chorionic blood vessels they both develop normally and there is 

 no free-martin produced. Furthermore, it was shown that the 

 testis in the male calf develops more rapidly than the ovary 

 in the female, and so may be supposed to exercise its influence 



