SEX IN ANIMALS 



539 



Sex-determination in Birds. Contrary to the condition in mammals, 

 in birds the determination of sex appears to depend upon the WZ type of 

 sex-inheritance, for in them the female is heterozygous for the sex factor, 

 and the male homozygous. The cytological evidence for this conclusion, 

 while limited to investigations on the domestic fowl, has through the 

 persistent and painstaking efforts of Guyer, been very definitely if not 

 quite completely worked out. In this species there are 18 chromosomes 

 in the somatic and primary germ cells of the male and 17 in the female; 

 in the former there are two sex-chromosomes and in the latter, one. 

 The eggs, therefore, are of two classes, those possessing a sex-chromosome 

 and hence male-producing, and those lacking it and hence female-pro- 

 ducing. In the male both the sex-chromosomes pass together during the 

 reduction division into one daughter cell, so that half of the sperm are 

 provided with a sex-chromosome and half are not. The fate of the 

 sperm lacking a sex-chromosome is still somewhat in doubt, but there is 

 considerable cytological evidence indicating that part of the developing 

 sperm degenerate. Moreover, the fact that statistical examination of 

 head length of the spermatozoa reveals only one class of sperm certainly 

 strengthens this inference. Thus the cytological evidence is in complete 

 harmony with the extensive evidence from sex-linked characters in this 

 species. 



The mode of inheritance of the barring factor in the domestic fowl 

 has already been described in detail. Other factors in the domestic fowl 

 which have been found to exhibit sex-linked inheritance are those for 

 inhibition of pigmentation in the silky fowl, and of red in the plumage of 

 the Columbian Wyandotte and gray in the White Wyandotte, also a 

 factor for high fecundity. In pigeons, also, it has been found that the 

 factor for dilute pigmentation is sex-linked, and follows the same type of 

 distribution as that described for the domestic fowl. 



The Sex-ratio. The Mendelian theory of the inheritance of sex 

 stipulates that in the heterozygous sex male and female producing 

 gametes are formed in equal numbers ; for in every reduction division one 

 pole of necessity must con- 



tain the accessory chromo- 

 some or sex-determining 

 factor whereas the other 

 must either lack it or con- 

 tain its unequal homologous 

 chromosome or sex-deter- 

 mining factor. As a con- 

 sequence of this fact we 

 should expect the sex-ratio 

 to show an approximate 



TABLE LXX. Sex Ratios in Animals. 



