EFFECTS OF GONAD GRAFTS IN CHICKS 35 



acterized by the differentiation of the seminiferous tubules in 

 the male and the cortical cords in the female, by the retrogression 

 of the miillerian ducts in the male and the degeneration of the 

 right ovary and duct, and differentiation of the left duct into an 

 oviduct in the female. 



As a consequence of the developmental time relations with 

 reference to sexual differentiation, it is reasonable to expect 

 that the grafts will produce the greatest effect during the second 

 week of incubation. This period is the period of sexual differ- 

 entiation; the sexual characters have not yet become fixed, 

 and hence it is reasonable to suppose that they may be con- 

 trolled by introduced factors. It is probable that the grafts 

 would be still more effective if they could be implanted before 

 sexual differentiation begins, that is, on the fourth or fifth day; 

 but as already stated, grafts do not grow on such early embryos, 

 owing to the fact that the allantoic circulation has not yet been 

 established. 



/. Degree of reversibility. Examples of natural or experimental 

 reversals of sex have already been quoted. Such cases are those 

 of the free-martin (Lillie), the guinea-pig and rat (Steinach, 

 Moore), Bonellia (Baltzer), and Crepidula (Gould). Inter- 

 sexes or sex intergrades have been obtained in pigeons by Rid- 

 dle, in the gypsy-moth by Goldschmidt, in Simocephalus by 

 Banta ('16 a, b), and in Drosophila by Sturtevant ('20). In 

 these cases various degrees of reversal of sex were noted, and 

 this reversal was correlated in a quantitative way with the degree 

 of development of the sex organs. The cases of Lillie, Steinach, 

 and Moore are the only cases in which the sex reversal was 

 explained as the direct result of the action of sex-hormones 

 produced by the gonads. 



The present experiments bear upon the problem of the reversi- 

 bility of sex. Intersexual conditions were produced in chick 

 embryos by grafting pieces of testis and ovary upon their mem- 

 branes. The interpretation of the intersexual individuals is 

 rendered very diflficult in the case of experiments of this kind, 

 since we do not with certainty know the sex of the embryos on 

 which the grafts were implanted. In other words, we do not 



