The epididymis is an internal reproductive organ in males that transports and stores sperm cells that were first produced in the testes. The epididymis is also the site of sperm maturation.
Sperm that are produced in the testes are not fully developed and are not able to fertilize an egg yet. In the epididymis the sperm complete their development and fully mature. When the male is sexual aroused, the epididymis transports the fully matured sperm cells to the vas deferens. From the vas deferens, the sperm is transported to the ejaculatory ducts and is then ejaculated out of the penis.