SEXUAL CHARACTERISTICS OF THELIA 623 
544 that each sex possesses the genes for the secondary sexual 
characteristics of the opposite sex, but that normally one set 
finds expression in the male and the other set in the female. 
Thus they differ from sex-linked characteristics which may be 
present in either males or females, although this distinction is 
sometimes not recognized. The ‘exclusively male character- 
istics’ studied by Foot and Strobell (15, ’17 a, 717 b) belong to 
the secondary sexual characteristics which in the cases best 
known have their genes located in the autosomes. The above- 
named investigators have shown in their Euschistus crosses that 
the gene or genes controlling the length of the intromittant 
organ may be transmitted from father to son, and that, since 
the son receives no x-chromosome from the male pronucleus, 
this gene or group of genes must be located in the autosomes. 
It does not seem improbable or impossible that the expression 
of a particular set of genes for the secondary sexual character- 
istics, even though located in the autosomes, should be dependent 
upon some controlling gene or genes in the x-chromosome. 
With the knowledge of the multiple effects of a single gene, and 
the presence of modifying and inhibiting factors, more and more 
evidence is being accumulated to show that a particular factor 
in one chromosome may influence the expression of genes in 
other chromosomes. The writer would contend that the impor- 
tant and vital genes of the x-chromosomes, not those forming the 
sex-linked characteristics, but those which must be present 
either once or twice for development to take place at all, create, 
if present once, an environment for a series of changes leading 
to the expression one by one of the male characteristics, and, if 
present twice, lead to the development of the female character- 
istics. If we consider for a moment the ontogenetic series in 
Thelia, we find that in the embryo one may see the differentia- 
tion of the gonads into ovaries or testes. The arrangement of 
the gonia, the form and method of attachment of the tubules, 
the structure of the gonaducts, and the position of the genital 
apertures early distinguish the males from the females. The 
external genital appendages, developing most probably from the 
primitive abdominal limb buds of the eighth to tenth somites, 
