THE LIMITS OF HEREDITY 201 



sterile female in cattle has thrown a good deal of 

 light on questions of sex alteration, and also on the 

 production of twins in cattle. John Hunter (1779), 

 in an early account of the free-martin, states that it 

 was a type well recognised b}^ the farmers at that 

 time. Such barren cows in Roman times were called 

 taurce, but apparently the Romans did not understand 

 the manner of their production. Hunter says: '* It 

 is a known fact, and, I believe, is understood to be 

 universal, that when a cow brings forth two calves, 

 and that one of them is a bull-calf, and the other a 

 cow to appearance, the cow-calf is unfit for pro- 

 pagation; but the bull-calf becomes a very proper 

 bull." This clearly states the general facts regarding 

 the production of free-martins. Hunter dissected 

 three such animals and considered them hermaphro- 

 dites, but found that they differed from one another. 

 He gives the anatomical details of his dissections, 

 and states that free-martins are " much larger than 

 either the bull or the cow, and the horns grow longer, 

 being very similar to the horns of an ox." 



The important investigations of Lillie (191 7) on the 

 foetal development of twins in cattle has furnished 

 a clear explanation of the fact that the free-martin 

 is nearh^ always sterile. The internal reproductive 

 organs of the free-martin are found to be usually 

 predominantly male, while the external organs are 

 at least usually female, but there are considerable 

 variations. Lillie 's main disco verv was that in the 

 production of twin embryo cattle there is a secondary 

 fusion of the two chorions and anastomosis of the 

 circulation of the two foetuses. The free-martin is a 

 female transformed by the action of hormones derived 

 from the male twin, which pass through the anasto- 

 mosed bloodvessels into the circulation of the female 

 embryo. Nearl}- all twins of cattle are monochorial, 

 with the chorions more or less completely fused. 



