Many experiments have demonstrated that varying the endocrine milieu makes it possible to act on sex determination; other grafting and castration experiments carried out on adult animals have led to the modern theory of sexuality: in male and female vertebrates, the soma is identical and can be considered a neutral element; the action of the gonad gives it its sexual characteristics; some of the secreted hormones act as stimulants and others as inhibitors; the genital tract itself is somatic, and embryology shows that it takes shape under the influence of hormones from bisexual precursors.