196 PHYSICAL BASIS OF HEREDITY 



that the mode of development of the embryo precludes 

 this interpretation. 



Riddle obtained his intersexual hybrids by causing 

 their mother to produce many more eggs than she would 

 ordinarily produce. This was done by removing the eggs 

 from the nest as soon as they were laid. Towards the end 

 of a series obtained in this way an overworked female 

 produced an excess of males. Some of these males Riddle 

 regards as females that have been changed into males — 

 the completeness of the change being shown in their sexual 

 behavior towards other males, etc. But there is involved 

 in the cross a sex-linked factor that behaves, as R. M. 

 Strong had already shown several years ago, as do sex- 

 linked factors in other birds. It is thus possible to identify 

 the chromosomal make-up of Riddle 's intersexual hybrids. 

 His own results show that the hybrids have the expected 

 combination of chromosomes for males. It appears, there- 

 fore, that whatever it may be that affects their behavior 

 their sex is determined by their possessing the ordinary 

 chromosome constitution for males. 



Hermaphroditism and Sex 



As has been shown, the sex-mechanism, whether XX- 

 XY or WZ-ZZ, gives rise to two kinds of individuals — 

 males and females. There are, however, many groups and 

 species of animals where both eggs and sperm are found 

 within the same individuals, and in typical cases there are 

 in such individuals special ducts that are outlets for the 

 male germ-cells and others for the female germ-cells. In 

 these hermaphrodites '^sex-chromosomes^' are not known 

 to be present, or if present as in Ascaris nigrovenosa, they 

 act as sex determinants only in alternate generations. 



The usual interpretation of the determination of the 

 sex-ceUs of hermaphrodites is that their differentiation 

 is determined by the same kind of specific influences that 

 determine, for example, that certain cells of the primitive 

 gut develop into liver cells, others into lung cells, still 



