Ancient myths of an hermaphrodite origin of man and his subsequent bisection into male and female individuals receive unexpected confirmation from the embryological evidence. The genital ridge contains the components to form either a testis or an ovary, while two sets of genital ducts are the forerunners of the male and female reproductive tracts. The presence of potentially hermaphrodite rudiments in different organisms resulted in the failure of attempts to formulate the determination of sex in terms of classical genetics. This, in turn, has led to the hypothesis that the function of the mammalian Y chromosome is to enhance the growth of the gonadal rudiment in male embryos. The hypothesis provides an explanation for the observed bilateral asymmetry of gonadal differentiation in human hermaphrodites in terms of the bilateral asymmetry of growth of human fetal gonads. The human condition can be related to the marked asymmetry of gonadal growth and differentiation in birds. The effect of temperature in the determination of sex in alligators has likewise been explained in terms of gonadal growth. The variety of sex-determining mechanisms met with in vertebrates suggests a non-sex-specific effect adapted to specific biological needs. It is suggested that certain DNA sequences on the human Y chromosome act by enhancing the growth of somatic cells in the gonadal rudiment, leading to precocious differentiation of the testis.