316 GENERAL EMBRYOLOGY 



female produces eggs which if fertilized produce females, if 

 unfertilized produce males. 



One further point is to be mentioned in connection with 

 the general hypothesis of nuclear determination. This is in 

 connection with that curious form of Mendelian heredity 

 known as " sex-limited" heredity, where certain characters 

 are exhibited by the individuals of one sex only, although 

 transmitted by the individuals of the other sex without being 

 exhibited by them. Such a form of heredity can readily be 

 explained upon the chromosome hypothesis, upon the simple 

 assumption of a close relation between the " determiner" for 

 the sex-limited character and the sex-determining element. 



Any further consideration of the problems of sex determina- 

 tion and heredity would lead us too far afield; more extended 

 treatment must be had from other sources. In conclusion 

 then, it is hardly necessary to point out that the constancy of 

 the form and of the complicated behavior of these idiochromo- 

 somes affords very striking confirmation of the hypothesis of 

 the specificity and genetic continuity of the chromosomes. 

 While it is possible that their form and behavior are determined 

 by underlying conditions, such conditions cannot be directly 

 observed and can only be postulated. Taken in connection 

 with the facts mentioned in Chapter II, and with the results 

 drawn from the development of dispermic eggs, and from 

 hybridization, they amount to practical demonstration of 

 some form of chromosomal specificity in development. 



As to the question whether the idiochromosomes are in 

 particular the sex determinants, several views may be held in 

 the absence of conclusive experimental demonstration of the 

 precise relation. It has been held in some quarters that sex 

 is determined by the relative amount of chromatin received 

 into the nucleus of the zygote, irrespective of its content in 

 certain chromosomal elements. This is hardly tenable how- 

 ever in view of many contradictory conditions. Others have 

 suggested that the dimorphism of the gametes is merely asso- 

 ciated with other more fundamental diversities, and that sex 

 differentiation and gamete differentiation are related only be- 



