Sperm are produced in the testes and move into the epididymis where they mature over a period of 10 to 14 days. Sperm are stored here and in the vas deferens where they can remain viable for several months. When ejaculation occurs muscle contractions of the vas deferens propel sperm into the ejaculatory duct where is is mixed with seminal fluid from the seminal vesicles. This fluid constitutes about 60% of the volume of semen. The sperm and seminal fluid continue through the prostate gland into the (prostatic) urethra. Here it is mixed with prostatic secretions which enter the urethra via many tiny prostatic ducts. Next the bulbourethral glands contribute fluid to the semen as it exits the prostate and continues through the urethra. Lastly muscles near the bulb of the penis help to propel the semen through the urethra out of the body.