THE DETERMINATION OF SEX 335 



long curved tail feathers characteristic of most cocks. In this 

 case a type of cell usually found only in ovaries is present in the 

 testes. It apparently produces a hormone which influences the 

 development of the tail feathers and the removal of the testes 

 or ovaries in which it occurs frees the individual from this influence. 



When we consider that the males of mammals are heterogametic 

 while in the birds this condition is true of the opposite sex, the 

 contrasting effects of castration are seen to be quite in harmony 

 with the relationship of chromosomes and sex. 



Freemartins afford another striking illustration of the effect 

 of hormones. They are infertile female cattle, twin-born with 

 males. Lillie has concluded in his studies of twinning that 

 the condition is due to the mingling of l)lood in the two foetal 

 circulatory systems through their close approximation in the 

 maternal uterus. The hormones produced in either body would 

 be carried through l)oth by the fusion of the systems. Those 

 produced in the male appear to inhil)it the normal development 

 of the female, with resulting sterility, although they are not 

 capable of bringing about complete reversal of sex. 



The indifferent stage of development through which all indi- 

 viduals pass, and the fact that the primary sexual organs of the 

 two sexes show definite homologies still further indicate that l^oth 

 male and female have a common l^eginning. The abnormal 

 cases cited show that residual capacity for the expression of one 

 sex may remain even after the complete differentiation of its 

 opposite. In these cases it is evident also that the differentiation 

 of a heterogametic sex is an advance over that of a homogametic 

 sex. 



Sex Reversal. Man has not yet succeeded in determining the 

 sex of animals under his control, but the facts so far presented 

 suggest that such control might be possible, and records of sex 

 reversal in fishes, amphibia and fowls show that the chromosomes 

 are not an insurmountable obstacle. One of the most striking 

 instances of sex reversal on record is that published by Crew in 

 the Journal of Heredity. This case records that an adult hen 

 from whose eggs normal chicks had hatched began to show the 

 secondary sexual characters of the male and later became the 

 father of chicks (Fig. 190). As in other cases of apparent sex 

 reversal in fowls and pigeons this change was due to the destruction 

 of the ovary by tuberculosis. Crew states that fresh growths of 



