SEX-CHARACTERS IN EVOLUTION 115 



it is thus of no more importance than the absence 

 of an X chromosome which occurs in those cases 

 where the male has one sex - chromosome and 

 the female two. According to the researches of 

 von Winiwarter 1 on spermatogenesis in man, the 

 latter is actually the case in the human species. 

 This investigator found that there were 48 chromo- 

 somes in the female cell, 47 in the male ; after 

 the reduction divisions the unfertilised ova had 

 24 chromosomes, half the spermatids 24 and half 

 23, so that sex is determined in man by the 

 spermatozoon. 



Morgan believes that the heredity of haemophilia 

 (the constitutional defect which prevents the spon- 

 taneous cessation of bleeding) follows the same 

 scheme, and also at least some forms of stationary 

 night-blindness that is, the inability to see in 

 twilight. 



We may mention a few other cases in animals, 

 referring the reader for a fuller account to the works 

 cited. One example is the barred character of the 

 feathers in the breed of fowls called Plymouth Rock. 

 In this case the female is heterozygous for sex as in 

 Abraxas grossulariata, and the barred character is 

 sex-linked. When a barred hen is crossed with an 

 unbarred cock all the male offspring are barred, all 

 the females plain. On the other hand, if a barred 

 cock is crossed with an unbarred hen, the barred 

 character appears in all the offspring, both males and 

 females. The female thus transmits the character 

 only to her sons. If we represent the barred char- 

 acter by B y and its absence by 6, we can represent 

 the heredity as follows : 



1 ' Spermatogen^se humaine,' Arch, de Biol, xxvii., 1912. 



