The Relation Between the Development of the Secondary Sex Characters and the Structure of the Testis in the Teleost <i>Xiphophorus Helleri</i> Heckel

Abstract

ABSTRACT The aim of this study was to trace the relation between the development of the secondary sex characters and the structure of the testis in the Cyprinodont fish Xiphophorus Helleri Heckel. The secondary sex characters of both male and female are described. Only the development of the most conspicuous sex characters, viz., the gonopod, i.e., the anal fin transformed into an intromittent organ, and the sword, i.e., the enormously elongated ventral part of the caudal fin is considered in relation to spermatogenesis. These secondary sex characters are formed in the young male and are present throughout further life. The testis of the young immature male in which the secondary sex characters have not yet developed, contains only a small number of primary spermatogonia besides many tubules derived from the peritoneal epithelium. Simultaneously with the beginning of the transformation of the anal fin into an intromittent organ spermatogenesis starts, and at the same time the epithelium of these tubules transforms into germ cells, being situated in small cysts. When the spermatogenesis has reached the formation of spermatocytes the elongation of the caudal fin has generally started. The testis of the male in which the gonopod is fully developed, but of which the sword has not reached its maximum length, contains all stages of spermatogenesis up to spermatophores, but the latter are still found in the cysts. In the fully mature male the spermatophores are lying in great numbers in the sperm ducts. The question as to the seat of the secretion of the sex hormones in the sex gland is discussed briefly. It appears that it is improbable that there is in the gonad a group of cells, being the specific source of the sex hormones.