The epididymis is a coiled tubular structure (~6 m when uncoiled) lying posterior to each testis. Sperm from the seminiferous tubules are immature and non-motile; transit through the epididymis (2–12 days) confers motility, forward progression capacity, and the ability to fertilize an egg. The vas deferens (ductus deferens) is the muscular tube that propels sperm from the epididymis toward the urethra during ejaculation.