ACTION OF SEX HORMONES IN FOETAL LIFE 383 



of these 38 were cf' cf , 67 cf 9 and 34 9 9 , thus an exceedingly 

 close approximation of the expected 1:2:1 ratio. In his "Prob- 

 lems of Genetics" Bateson cites Bernadin (La Bergerie de Ram- 

 bouillet, 1890, p. 100) as to the frequency of twin combinations 

 in Merino Sheep, viz: 87 d" d" to 187 d" 9 to 83 9 9, which also 

 approximates the expected 1:2:1 ratio. These statistics, there- 

 fore, also support the interpretation of the free-martin as female, 

 for they show that the actual ratios of the distribution of sex 

 among twins are as a matter of fact the expected ones in 

 ungulates. 



On the other hand in man there is a very significant and inter- 

 esting departure from the expected ratio: Simpson ('44) collected 

 statistics of 788 cases of human twins, the various sex combina- 

 tions being d^ d^ 229, o^ 9 298, 9 9 261, thus very far removed 

 from the 1:2:1 ratio. Nichols ('07) has made a very much 

 larger collection of statistics with the following ratios d d^ 

 234,497, cf 9 264,098, 9 9 219,312. It is obvious that there 

 is a very large disturbing factor here; this is almost certainly 

 the factor of monozygotic twinning. As the two-sexed combi- 

 nation must be dizygotic, we may estimate the dizygotic d^ d^ 

 and 9 9 combinations at one-half of the d^ 9 combination, on 

 the slightly inaccurate basis of a 1 : 1 sex ratio of male and female 

 zygotes. This would give 132,049 dizygotic d^ d^ and 9 9 t\vin 

 pairs each, and the excess viz : 102,448 d^ d^ and 87,263 9 9 would 

 represent the monozygotic twin couples. This is not very far 

 from the proportion of monozygotic twin pairs among one- 

 sexed twin couples estimated by physical resemblance. If, 

 then monozygotic twinning is the disturbing factor in the un- 

 expected sex-distribution ratios of human t\Adns, we may argue 

 from the fact that the ratios in cattle and sheep approach ex- 

 pectation that monozygotic twinning either does not occur, or is 

 very rare in them, and this is confirmed by the embryological 

 evidence. This matter is discussed more fully in Newman's 

 book on twins ('17) to which, the reader is referred. 



