500 
Katharine Foot and E. C. Strobell 
breeding of fowls have demonstrated that it is the female, not the 
male, that is heterozygous for sex. 
Arkell’s and Davexport’s (’12) theoretical Interpretation of the 
results of Crossing horned with hornless sheep is a case in which the mor- 
phology of the chromosomes is a pure assumption, for they assume the 
female has two X chromosomes, and the male the odd X chromosome, 
w’hereas these all important cytological details are still undetermined. 
Front the point of view of the cytologist, it is disappointing that so 
niiich of the experimental work on the transmission of “sex-linked cha- 
racters” has been done on material in which the morphology of the 
chromosomes is a very uncertain factor. The recent important dis- 
covery that in some forms it is possible to denionstrate a morphological 
difference between the chromosomes of the male and female has 
strengthened the hope that the chromosome theories might be put to 
the test of practica! experimental breeding, and it is probably due to 
this important cytological discovery that so much valuable work has 
been done on the transmission of “sex-linked characters”. 
But the value of this work as a practical test is forfeited if the mor- 
phology of the chromosomes is unknown, or even doubtful, and this im- 
portant factor must be supplied as a pure assumption. This difficulty is 
avoided in Euschistus. The morphology of the chromosomes of vario- 
larius and serms can be demonstrated by photographs as clear as those 
of the genital spot. 
Euschistus variolarius offers a still greater advantage, for the spot 
on the male genital segment is more than a sex-limited character, in the 
sense in which this term is frequently used by authors. As far as we are 
aware, scarcely any of the sex-limited characters which have been put 
to the test of experimental breeding are exclusively of one sex, for in 
some generations they appear in the opposite sex also, for example, Dox- 
caster’s pioneer experiments with Ahraxas, the cross-brceding experi- 
ments with horned and hornless sheep, the transmission of spurs in fowls, 
the sex-linked characters in Drosophila etc. The spot on the male genital 
segment of variolarius, on the contrary, never appears in the female, 
and it therefore cannot be classed vith the sex-linked characters, nor the 
secondary sexual characters that can be inherited by both sexes — it 
is a character that is part of the male genital segment and is as exclu- 
sively male as is the genital segment itself. This spot, therefore, would 
seem to be an exceptionally favorable character for testing the hypo- 
thesis that factors essential to the determination of sex and sex-limited 
characters are carricd by “sex chromosomes”. 
