SUPPLEMENTARY NOTES ON TWINS IN CATTLE. 49 



have failed on account of their small size, but it seems more prob- 

 able that they represented secondary connections formed too late 

 to be effective. 



We thus have twelve cases on record in which the female of 

 two-sexed twins of cattle is normal, and in which the conditions 

 of the fcetal membranes are also known — six cases of Keller and 

 Tandler and six cases of my own. In nine of these cases there 

 was certainly no vascular connection; two of my own cases were 

 not satisfactorily investigated with reference to this point, and in 

 one of Keller and Tandler's cases the fine anastomoses that existed 

 were apparently secondary. In all other cases of two-sexed calf 

 twins in which the fcetal blood vessels were examined (29 of the 

 author and 85 of Keller and Tandler) there was anastomosis be- 

 tween the fcetal vessels and the female was definitely intersexual. 



The proportion of cases in which the female of two-sexed twins 

 in cattle is normal was found by Keller and Tandler to be 6 out 

 of 91 ; in my collection it is 6 out of 39 ; Numan estimates the 

 proportion to be 1 in 8, and Liier (191 3) 6 in 113. (Cited from 

 Keller and Tandler.) 



The experiments of Minoura (1921) in which he grafted pieces 

 of testis or of ovary on the allantoic membrane of chick embryos 

 and obtained as a result intersexes of various grades, furnish com- 

 plete proof that sex hormones can produce precisely the same kind 

 of changes that are found in the free-martin. 



2. The Sex-ratios. 



The original account involved 54 cases of foetal twin calves 

 in which the distribution of the sexes was as follows : $ $ 19, 

 55 3, H 21, ? ? 1 1 ; or classifying the normal and intersexual 

 females of two-sexed pairs together, $ $ 19, $ $ 24, 2 $ 11. The 

 expected ratios, assuming approximate equality of male and female 

 zygotes in cattle, would be in the proportions of 1:2: 1 — i.e., 13.5 : 

 27 : 13.5. It is obvious that there is an excess of male pairs which 

 is not readily interpretable as due to chance, even, though the num- 

 bers are relatively small. 



I have now 38 additional pairs of fcetal twin calves (Nos. 58-97, 

 excluding No. 66, which was a pair selected after birth, and No. 80, 



