During month 3, the prostate develops from the prostatic part of the urethra (endoderm of the canalis vesico-urethralis and mesoderm of the sinus urogenitalis definitivus). The first 14 to 20 buds are of endodermal origin and form the outer zone of the prostate. The buds arising later are of mixed origin (urogenital sinus, mesonephric duct and eventually also the paramesonephric duct). They form the inner zone. The buds are initially solid. They later become tubular, and penetrate the surrounding mesenchyme.