RELATION TO GENERAL BIOLOGICAL PROBLEMS 117 



martin. Here an individual zygotically determined 

 to be a female twin may become more or less com- 

 pletely differentiated into a male by the very neat 

 device of borrowing hormone-charged blood from its 

 male co-twin. 



If two male twins mutually transfuse blood, no 

 alteration of the sex-bias occurs. The same is true 

 in the case of two female twins. But wherever the 

 twfns are opposite-sexed, the female is the one to suffer 

 sex-reversal. At first it was not clear to Lillie whether 

 this result was due to a dominance of male-differentiating 

 hormones over female, or was the result of a precocious 

 development of male glands. Further studies favor the 

 latter interpretation, and it now appears that the glan- 

 dular portion of the testis differentiates before that 

 of the ovary and that w^hen the twins unite blood 

 vessels in the chorion, the blood of the male passes into 

 the system of the female and inhibits the development 

 of the glandular portion of the ovary. In the absence 

 of female-differentiating hormones in the zygotically 

 determined female, the male hormones have full play 

 and actually do cause the differentiation of male charac- 

 ters in the freemartin. The freemartin is then a paradox : 

 it is a zygotic female, but differentiated more or less 

 extensively into a male. 



These results make it necessary then to distinguish 

 most carefully between sex-determination and sex- 

 differentiation. Sex may be determined zygotically, 

 but may be altered more or less completely by a change 

 in the hormones. The chromosome mechanism appears 

 to fix the sex-bias and the hormone mechanism to 

 bring about sex-differentiation. Normally, however, 



