SEXUAL DIMORPHISM IN THE PECTORAL FIN OF GAMBUSIA AND THE INDUCTION OF THE MALE CHARACTER IN THE FEMALE BY ANDROGENIC HORMONES

¹ Department of Zoology, Northwestern University, Evanston, Illinois

1. The pectoral fins of juvenile males and females in Gambusia are practically identical in structure.

2. In Gambusia and in a number of closely related genera a modification of the pectoral fin occurs in the male with the onset of sexual maturity. The structural peculiarities of the character consist of elongation and segmentation of rays 2, 3, 4, and of the anterior branch of ray 5, and the partial separation of the dorsal and the ventral components of these rays, a general widening of the ventral component and a development of an imbricated area by further widening of the ventral components within a localized area.

3. Females possess one or more genetic factors for the same structural characters and they can be induced to develop the characters by the application of methyl testosterone or ethynyl testosterone.

4. The first stages in the development of the characters are induced in medium sized females by treatment with methyl testosterone at a concentration of 1 mg./20,000,000 cc. of water. A concentration of 1 mg./5,000,000 cc. produces maximal development of the characters. Ethynyl testosterone produces the same results at concentrations a little more than twice as great.