An interactive, animated Flash diagram of duct differentiation. Before the seventh week of embryonic life both girls and boys have two sets of urogenital ducts (tubes), called the Müllerian (or paramesonephric) ducts and Wolffian (or mesonephric) ducts, and both open into the back of the developing bladder, located close to the developing gonad. In a typical male fetus, cells in the testes make a hormone called MIS (Müllerian Inhibiting Substance), which makes the Müllerian ducts disappear, while other cells make the hormone testosterone to make the Wolffian ducts develop into spermatic ducts. In a female embryo, there are no testes to produce MIS or testosterone, and as a result the Wolffian ducts disappear and the Müllerian ducts become the uterus, fallopian tubes, and part of the vagina. Click through a slide-show to see the process in action.