SEXUAL CHARACTERISTICS OF THELIA 619 
active, an unessential in the male, and that males might be 
produced lacking this element, yet no zygote in the ‘non-dis- 
junction’ experiments formed by the union of an ovum minus 
an x-chromosome and a sperm also without this chromosome 
developed. Likewise, ova containing two x-chromosomes never 
developed if fertilized by spermatozoa containing an x-element. 
Thus females with three x-chromosomes are not formed. These 
same experiments of Bridges also demonstrated that the y-chro- 
mosome is without influence in sex production for xxy females 
and xyy males are in no way different from normal individuals 
except in the types of gametes produced. No genes have been 
located in the y-chromosome, yet from its behavior in sper- 
matogenesis it seems to have some affinity for the x-chromo- 
some, and it is not improbable that the y-chromosome represents 
an altered x-chromosome rendered inactive through some change 
in its makeup. Its affinity for the x-chromosome is shown in 
the formation of an unequal xy tetrad in the first spermatocyte 
division, as in the Coleoptera, or an unequal xy diad in the second 
spermatocyte, as in the Heteroptera. In Enchenopa binotata 
(Kornhauser, 714) these two chromosomes unite end to end in 
syndesis and form a tetrad composed of two elements similar in 
size. ‘Thus, I believe, we are to look upon the y-chromosome as 
having originally been the partner and homologue of the x-chro- 
mosome, but that a change, perhaps the loss of a single gene, 
made it inactive as a carrier of genes. Inactive and therefore 
unimportant, it might undergo many chance variations or losses 
which might culminate in the final disappearance of the y-element, 
a condition not at all uncommon in the insects. Bridges (’17) 
has recently shown that a chromosome may become deficient 
as a bearer of genes. A race of flies was produced in which the 
x-chromosome was abnormal in that a particular ‘measurable 
section of genes’ was either inactivated or lost. This experi- 
ment further demonstrates that ‘deficient’ x-chromosomes pro- 
duce normal sex ratios, and he concludes that the determiner of 
sex is not the ‘x-as-a whole,’ but that in some definite part or 
parts of the x there are specific sex-differentiators. 
