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- TasLp LXX. Sex Ratios in Animals. 
Song <0r sex-determining Animal | Males | Females Authority 
factor whereas the other | 
must either Jack it or con- Horse. 98.3 100 Dising 
tainitsunequalhomologous Cattle ....|_ 107.3 100 Wilckens 
chromosome or sex-deter-  Sheep..... 97.7 100 Darwin 
mining factor. As a con- Swine..... 111.8 100 | Wilckens 
> 1 REM Fan ronctetae 105.0 100 Cuénot 
Seas Olathe itact ie Dove..... 105.0 100 Cuénot 
should expect the sex-ratio ae 04.7 100 Taree 
to show an approximate 
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