The Cliromosomes of Euschistus variolarius, Euschistus servus etc. 499 
the responsibility of suppressing the spot factors in all tlie females, but 
also of cletermining just how many spot factors shall find expression in 
the males of the Fi and F2 generations, and thus they practically re- 
lieve the chroniosomes of the bürden of unit distribution.” 
The results of much of the experimental breeding of the past decade 
have been confidently offered in Support of the hypothesis that factors 
determining the transmission of unit characters are carried and distri- 
buted by definite chroniosomes. It is claimed that the transmission of some 
ofthe secondary sexual characters and “sex-linked characters” is explicable 
on the assumption that the factors essential to their determinatio aren 
linked mth the factors which determine sex, and that these factors are 
contained in a definite chromosome — the X chromosome. This hypo- 
thesis, offered in explanation of the transmission of sex-linked characters, 
is clearly stated by Morgan (TI). His fine experimental work on sex- 
linked characters in Drosophila led him to the above-quoted conclusion: 
“MTiat is most important is the discovery that the X chromosome con- 
tains not only one of the essential factors in sex-determination, but also 
all other characters that are sex-limited in inheritance.” 
Such extreme views of the function of the chromosomes in here- 
dity are hehl by only a few of the investigators who have experimented 
with the inheritance of sex-limited characters. Other students of genetics, 
v’hile accepting the hypothesis that the factors determining secondary 
sexual characters are hnked with the factors determining sex, do not 
definitely locahze them in the germplasm. These investigators treat both 
series of factors as purely hypothetical, the sex-linked factors being in 
some cases theoretically linked with the male — or female — determining 
factor, as the observed results of inheritance in their cross-breeding ex- 
periments seem to demand. In most of these cases the morphological 
facts of cytology are ignored, due to a scepticism of the existence of sex- 
determining chromosomes, or to the fact that the material is unfavorable 
for determination of the necessary cytological data. In the latter case 
the investigator who believes in sex chromosomes must simply assume 
his cytological facts. If the results of his experiments demand that the 
female be heterozygous for sex, the female should have the odd chromo- 
some, and woe to the cytologist who dares to find an odd chromosome 
associated with the wrong sex! 
A case in point is Guyer’s (’09) claim that the male chick has an odd 
(X) chromosome. This is embarassing to the cytologist who beheves 
that the presence of this odd chromosome is a proof that the male chick 
is heterozygous for sex, for it is claimed that the results of experimental 
