Illustration copyright 2000 by Nucleus Communications, Inc. All rights reserved. http://www.nucleusinc.com

Spermatozoa (sperm) are the male sex cells that carry a man's genetic material. A sperm fertilizes a woman's egg (ovum) by breaking through the membrane that surrounds the egg.

Sperm develop in a man's testicles and finish maturing in the epididymides. During ejaculation, the sperm move out of the epididymides through the vas deferens and into the urethra, the tube that runs through the penis. Semen, the thick liquid produced in the seminal vesicles and prostate gland, is added to the sperm before ejaculation.