Abstract
In this review of a particular line of work in the field of physiological genetics, the author summarizes the results of expts. on eye color development in Ephestia and Drosophila. These expts. are concerned with hereditary characters differentiated by genes producing their effects through the action of hormone-like substances that control specific reactions at definite developmental stages. These substances are not enzymes, fats or proteins but, according to the author, it is possible that they are closely related to the nucleic acid constituents of chromosomes.