ZOOLOGICAL SCIENCE 207 



Fig. 27 give rise to germ-cells having one member of each 

 pair of chromosomes. But in the male one-half of the 

 spermatozoa will be without an "X" chromosome, because 

 males possess only one member of this pair. In the female 

 (Fig. 27, Pi), each egg will, of course, have a single sex 

 chromosome, since both members of this pair are present in 

 the cells of females. There are, therefore, two kinds of 

 spermatozoa, (1) those with, and (2) those without the 

 "X" chromosome; but only one kind of ovum. When such 

 sperms and ova meet, with equal chances of union, there 

 will be, by the laws of chance, equal numbers of males and 

 females produced, as shown by the chromosome combination 

 in the Fi generation of Fig. 27. This agrees with the 

 observed fact that in many animals the numbers are equal 

 in the two sexes and explains how the formula for the cells of 

 a female comes to be 2n + 2x and for the male 2n + lx. 

 There has, therefore, been established for a considerable 

 number of animals, a theory of sex-determination, which 

 recognizes not only the germ-cells as the important item but 

 a specific chromosome within the germ-cells. This theory is, 

 therefore, a part of the general theory of chromosomes in 

 relation to heredity. 



Through the work of the geneticist, this recent cytological 

 theory of the determination of sex is correlated with facts 

 known for the inheritance of what are termed sex linked 

 characters. The peculiar inheritance of color blindness in 

 man, which has long been known to be related to the sex 

 of the individual, may thus be explained. Referring again to 

 Fig. 27, let us assume the existence of sex chromosomes in 

 the human body and its germ-cells, and assume further that 

 one of these chromosomes carries the factor for color blind- 

 ness. 19 The presence of such a factor may be indicated by 



19 The difficulty in obtaining satisfactory material makes the number of 

 chromosomes in man still a matter of doubt. Winiwater, H. v. (Arch, de 

 Biol., Vol. 27. 1912) has reported 48 in the female and 47 in the male, but 

 these results have been disputed. 



