170 Heredity and Environment 



this is true. In the normal fly of this species there are four pairs 

 of chromosomes (Fig. 65) ; the first pair (Chromosomes I) are 

 the sex chromosomes which are XX in the female and XY in the 

 male; the second and third pairs (Chromosomes II and III) are 

 large and V-shaped; the fourth pair (Chromosomes IV) are very 

 small and round. Through the failure of chromosome pairs to 

 separate in the maturation divisions an egg or sperm may come 

 to contain an abnormal number of any or all of these chromo- 

 somes. 



As opposed to the sex chromosomes, all others are known col- 

 lectively as "autosomes." A normal female has two X chromo- 

 somes and two of each of the autosomes ; a normal male has one 

 X and two of each of the others ; but when two X's occur with 

 three of each of the others, or with three of some of them, inter- 

 sexes result, and these may grade all the way from perfect females 

 to perfect males depending upon the ratio of X chromosomes to 

 autosomes. Sex is therefore determined by a quantitative rela- 

 tion of the X chromosomes to the autosomes, and if one assumes 

 that there are sex enzymes, such as Goldschmidt postulated, it 

 is probable that the X chromosomes produce mainly "gynase" 

 and the autosomes "andrase." Such an explanation harmonizes 

 well not only with all that is known regarding the chromosomal 

 determination of sex and of intersexes but also with much that 

 is known concerning the possibility of modifying the development 

 of sex by enzymes or hormones from glands of internal secretion. 



In either sex many secondary sexual characters of the other 

 sex are present during development and traces of these may per- 

 sist in the adult; but one set of these characters develops fully 

 in the male and another set in the female, so that they may be 

 called sex-limited. The development of the secondary sex char- 

 acters is usually determined by internal secretions from the ova- 

 ries or testes, though in some cases they may develop after these 

 organs have been removed, but in the last analysis both primary 

 and secondary sex characters are dependent upon the sex deter- 



