SUPPLEMENTARY NOTES ON TWINS IN CATTLE. 75 



sex hormones in this case. The sex hormones may be unbalanced 

 as against each other or as against genetic sex factors in other 

 ways than by anastomosis of fcetal circulations in different-sexed 

 individuals. As I pointed out previously, there may be a breaking- 

 down in the presumable defence mechanism of the placenta that 

 protects the embryos from the sex hormones of the mother. Pos- 

 sibly other causes may also exist which in the present state of 

 development of sex hormone theory can not be anticipated. 



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