624 SIDNEY I. KORNHAUSER 
early become distinguishable as male or female, and this dif- 
ference becomes more and more apparent as development pro- 
ceeds. In the fourth and fifth nymphal instars the somatic 
cells as a whole show physiological differences in that the females 
grow more slowly, but grow to a greater size. Last of all, with 
the final molt and sexual maturity, comes the expression of a 
host of secondary sexual characteristics. Pigmentation, size, 
form, and detail of the sclerites show various sexual differences 
and the gonapophyses exhibit their specific characteristics. The 
difference in metabolic pitch now becomes very evident as the 
female stores yolk material, while the more active male uses up 
his stored energy. Behavior differences connected primarily 
with the act of reproduction also appear in the imagos. 
In the insects we know of no hormones produced in the gonads 
either early or late in ontogeny, which, circulating in the blood, 
constitute a factor in the internal environment of the somatic 
cells stimulating certain genes to find expression and suppressing 
others. In vertebrates such hormones certainly exist, and are 
generally believed to be produced in the interstitial tissue of the 
testis or ovary. They are different in the two sexes, as shown by 
their effects upon developing somas. ‘These hormones must be 
produced through the influence of certain genes which are active 
in the interstitial cells. That the hormone may be modified by 
new genes has been demonstrated by Morgan’s (’17) experi- 
ments on hen-feathered Seabright cocks. In these birds the 
testis produces a hormone which prevents the development of 
the normal male plumage and causes a large proportion of steril- 
ity, according to Goodale (16). That the sex-hormone-forming 
genes of the male find their expression in the presence of one 
x-chromosome while those of the female depend on two x-chro- 
mosomes seems not unlikely. The hormones from the gonads 
and probably in some cases from other endocrine glands circulate 
in the blood and form a very important step in sexual develop- 
ment, creating at times a necessary factor for the expression of 
those genes which follow in their activity in the series of develop- 
mental changes. But while no definite hormone forms an im- 
portant step in the sexual development of insects, still each gene 
