270 THE MOSAIC STAGE OF DIFFERENTIATION 



size and other male-type characteristics. It would thus appear that 

 male-type development has been already determined in the spur- 

 rudiments of the young male chick, although these are still very 

 small. The spur-rudiments of the female chick, on the other hand, 

 are in a labile, undetermined state. Whether the determination in 

 the male has been eiTected by the testis hormone acting on the 

 rudiment is not known. Against this is the fact that female spurs 

 grafted to male hosts, and then after varying periods up to 24 days 

 replanted in the original (female) donors, do not show male-type 

 growth. Possibly only the embryonic rudiment can be sensitised by 

 male hormone. The alternative hypothesis of different reactivity of 

 ZZ (male) and ZO (female) tissues must also be included (fig. 126). 

 In contradistinction to this case, it is known that in many ver- 

 tebrate organs, exhibiting sexual dimorphism in size and other 

 characters, the sex-differences are only maintained so long as the 

 hormones responsible are acting upon them (see, e.g., Goldschmidt, 

 1923). The whole problem of the time-relations of determination 

 exerted by hormones, and of its reversibility, needs careful experi- 

 mental analysis. 



