67S THE PHYSIOLOGY OF REPRODUCTION 



animals cited has been called by Morgan the WZ type. It is 

 represented diagrammatically as follows : — 



WZ mating with ZZ 



The females are heterozygous for sex, and the males homozygous— 

 that is to say, there are two sorts of ova but one sort of spermatozoon. 



This type of sex-inheritance appears to be characteristic of 

 Lepidoptera and birds, while the XY type occurs in other insects, in 

 Myriapods and spiders, in ISTematodes, and among Vertebrates in all 

 Mammals and probably also in fishes and Amphibians.^ 



Parkes,^ as a result of an ingenious research on numerical sex- 

 inequality in man, has arrived at very clear conclusions in support 

 of the view that the male is heterozygous and the female homozygous 

 for the sex-determinant. He examined many dozens of genealogies, 

 and of these found six to be abnormal in having a proportion of 

 more than 1100 males to 1000 females. The total number of 

 individuals was 1792, of which 1001 were males and 791 were 

 females. When the female lines were separated from the male the 

 following results were obtained : — 



Male lines 1167 : males, 685 ; females, 482. 



Female lines 625 : „ 316; „ 309. 



It was thus found that the excessive male-bearing tendency 

 was vested exclusively in the male line, which indicates that the 

 spermatozoa carry the sex-determinant, and that the abnormal 

 ratio is not accounted for by selective fertilisation. 



It has been shown, however, that with some animals, whereas 

 a certain chromosome constitution is undoubtedly usually correlated 

 with a particular sex, it cannot be regarded as the efficient cause of 

 that sex, since it may under special conditions be overridden, and 

 the alternative sex produced (see pp. 663 and 693). 



' King states that only one X chromosome occurs in the male of the 

 Amphibian, Nectwnm {Anat. Recwd, vol. vi., 1912). Among Mammals Wodsedalek 

 has found two sorts of sperms in the horse and pig. The numbers of somatic 

 chromosomes are: horse, i 36, 9 38; pig, <5 18, ? 20 (&'oZ. Bvll., vols, xxv., 

 xxvii., and xxx., 1913-14-17). In man there is some dispute about the numbers 

 of chromosomes, but according to von Winiwarter there are forty-seven in the 

 male and forty-eight in the female {Arch, de Biol., vol. xxvii., 1912). (See p. 168.) 



2 Parkes, "Sex-Heredity, with Special Reference to Abnormal Numerical 

 Inequality between the Sexes," Science Progress, vel. xv., (April) 1921. 



