The Cellular Basis 169 



determination of sex is not irreversible predestination but a quan- 

 titative overbalance in the direction of one sex or the other" 

 which may later be changed. Somewhat similar conclusions had 

 previously been reached by Whitman and by Riddle regarding the 

 sex of pigeons, by Shull in the case of Lychnis, and especially by 

 Goldschmidt for the gypsy-moth. Goldschmidt supposes that 

 sex is determined by certain enzymes which he calls "andrase" 

 and "gynase" ; an excess of the former leads to the development 

 of males, an excess of the latter to females, and varying mixtures 

 of the two to varying intergrades or "intersexes." He assumes 

 that these enzymes are present in the "sex determining" chro- 

 mosomes at fertilization as well as in later stages and thus he 

 attempts to identify all sex determining factors with these "sex 

 enzymes." The difference between determination by chromo- 

 somes and by internal secretions, that is, between heredity and 

 development, is found chiefly in the time at which these 

 enzymes act. 



Morgan has shown that "gynandromorphs" or "sex mosaics" 

 are due to the irregular distribution or loss of certain "sex 

 chromosomes" owing to abnormalities in fertilization or cleavage. 

 In such cases one portion of the body shows male characters, an- 

 other portion female ones and a study of the chromosomes in 

 these regions shows that in the former the male combination of 

 chromosomes is present, in the latter the female combination. 

 This comes as near to a demonstration of the truth of the chro- 

 mosomal theory of sex determination as is possible. 



Finally, the most notable recent work on this subject is by 

 Bridges.* In his studies of the pomace fly, Drosophila melano- 

 gaster, he found intersexes whose genetical behavior was such as 

 to suggest that they had more or less than the usual number of 

 chromosomes. By means of breeding experiments as well as by 

 microscopical study O'f their germ cells he has demonstrated that 



*Bridges, C. B. Triploid Intersexes in Drosophila Melanog aster. 

 Science, Sept 16, 1921. 



