The Mullerian and the Wolffian ducts are separate sets of primordial tissues that transiently co-exist in embryos of both sexes (see EMBRYO). In female embryos the Mullerian ducts grow and fuse in the mid line, producing the FALLOPIAN TUBES, the UTERUS and the upper third of the VAGINA, whereas the Wolffian ducts regress. In the male the Wolffian ducts give rise to the VAS DEFERENS, the seminal vesicles and the EPIDIDYMIS, and the Mullerian ducts disappear. This latter development requires a functioning testis (see TESTICLE) from which an inducer substance diffuses locally over the primordia to bring about the suppression of the Mullerian duct and the development of the Wolffian duct. In the absence of this substance, development proceeds along female lines, irrespective of the genetic sex.