20 HEREDITY AND EUGENICS 



The history of the sex chromosomes has been clearly 

 shown in many animals, and in the fruit fly Drosophila 

 a large number of sex-linked mutations, determined 

 apparently by changes in loci of the X-chromosome, 

 have been studied. 



A brief account is first necessary of the sex 

 chromosomes as they apparently exist in man. 

 Although many observations have been made on the 

 subject, the facts are not 3'et known with certainty; 

 but the details are gradually becoming clear. It 

 appeared at one time that the negro had 24 chromo- 

 somes and the white man 48, but this apparent 

 difference ma}^ have been due to clumping of the 

 chromosome pairs in the process of fixation, so that 

 they looked like single chromosomes. Also in the 

 earlier accounts one or two more chromosomes were 

 found in the female than in the male, but later investi- 

 gators are agreed that there is a pair (XY) of sex 

 chromosomes which are distinguishable b}^ their shape 

 and behaviour from the other chromosomes. It 

 appeared from the studies of Guyer (1910, 191 4) 

 and of Montgomery (191 2) on human spermato- 

 genesis that the male negro possesses 22 chromosomes, 

 including 2 sex or accessory chromosomes. Mont- 

 gomery found that the accessories were irregularly 

 distributed in the reduction divisions. It was inferred 

 that the female number was 24. Von Winiwarter 

 (191 2), however, studying members of the white race, 

 found 47 chromosomes in man and 48 in woman 

 (oogonial divisions). Farmer, Moore and Walker 

 (1906), in examining pathological tissue (somatic cells) 

 presumably of white people, found usually 32 chromo- 

 somes, while Wieman (191 3) counted 33 to 38 chro- 

 mosomes in a human embr3^o the parentage of which 

 is not stated. More recently Wieman (191 7) de- 

 scribes human spermatogenesis with 24 chromosomes 

 in both negro and white, including an XY pair of sex 



