ABSTRACT
Few studies of sex differentiation in elasmobranchs exist. Chieffi's comparative work on Torpedo ocel/ata, T. marmora/a, and Scy!iorhinus canicula are the most complete, and his comparisons with other published accounts of sex differentiation (Picon 1962) show parallels. The undifferentiated gonad of elasmobranchs derives from two separate embryonic tissues, the cortex from the peritoneum, and the medulla from the interrenal blastema (Chieffi 1951, 1952a, 1955). In a typically vertebrate manner, the germ cells migrate into the cortex via the dorsal mesentery after differentiation from early primitive endoderm, segregating very early, possibly even before embryo formation (Beard 1902). As in amniotes, differentiation of the male precedes that of females. Sex differentiation of the genital ridge in males occurs as the germ cells migrate from the cortex to the medulla (Stage 19). The cortex then disappears, and the newly populated medulla is characterized by sex cord formation. At this time, Mullerian duct regression occurs in males. ln females, the germ cells remain in the cortex until the embryo has increased in size (Stage 20). The
cells then migrate into the medulla, where they remain stable. Formation of primary follicles does not occur until just prior to oviposition in Torpedo spp. (Stage 34).
