Once the sperm are produced, they travel to the epididymis where they mature. Once sperm cells are mature, they travel into the body through a tube called the vas deferens . The mature sperm are held in the upper portion of the vas deferens until they are ejaculated. An ejaculation occurs as the muscles around the prostate gland and the penis contract. These contractions push the sperm into the urethra where they mix with seminal fluid .
Seminal fluid is produced by the seminal vesicles and prostate gland.
This fluid mixes with the sperm and is called semen . Semen leaves the body through the urethra in several spurts of fluid. One sign that a boy has begun puberty is the ejaculation of semen. Sometimes an erection occurs as a boy sleeps and dreams. A sexually exciting dream can cause an ejaculation.