202 HEREDITY AND EUGENICS 



About 1 3 per cent, of free-martins (z.^., females twinned 

 with a male calf) are fertile. This happens when the 

 fusion of chorions does not take place, and there is 

 hence no opportunity for the male embryo to influence 

 the development of the female. It is also found that 

 there are always in cattle two corpora lutea present 

 when twins are born, and a single one at single births. 

 This shows that the vast majority, at least, of twins 

 in cattle are derived from two separate eggs. There 

 is no certain evidence that monozygotic twins ever 

 occur in cattle. This is contrary to the condition in 

 man, where both monozygotic (identical) and dizygotic 

 (fraternal) twins are now known to occur. 



That monozygotic twins occur but rarel}', if at all, 

 in cattle and sheep, is shown b}^ the ratio of the sexes 

 in twin births. This ratio closel}^ approximates 

 ic?c^:2?c^:i$ ?, which would occur if all tw^ins were 

 derived from separate ova. The data on this subject 

 have been considered by Go wen (1922). In human 

 twin births there is a wide departure from this ratio, 

 owing to the occurrence of " identical " twins derived 

 from a single ovum. Thus, in statistical records of 

 Nichols, the proportions of the sexes in human twin 

 births is males, 234,497; male and female, 264,098; 

 females, 219,312. This represents a ratio of 

 I -07 : I -20 : I . These statistics are clearly in harmony 

 with the view^ that both uniovular and biovular twins 

 occur in man. 



Gowen has also compared the colour markings of 

 cattle twins, taken from the Herd Books of the 

 American Jersey Cattle Association. These records 

 included 749 twin females, 168 twin males, and 207 

 male and female. Compared as regards colour mark- 

 ings, tongue colour, and switch colour, there was found 

 to be little if any significantly greater resemblance 

 in favour of twins of like sex. There w^as, however, 

 5 per cent, greater resemblance in favour of male 



