In the male, semen is the fluid expelled during ejaculation. In addition to plasma, the semen ejaculate contains secretions from the seminal vesicles and other glands to support and nourish the living sperm cells (spermatozoa) contained within the semen. Sperm cells are haploid sex cells of the male. Unlike eggs (oocytes and the mature ovum) that are large, non-motile, and generally ovulated one at a time, sperm are tiny, motile, and produced in the millions. While the human sperm contain a relatively long tail (flagella), the volume of an entire sperm, tail and all, is only 1/85,000 of the mature ovum.

Reproduction in humans may occur when semen—containing sufficient numbers of living and healthy (viable) sperm cells--is deposited in the vagina of a female near the cervical opening of the uterus. The haploid sperm move through the cervix, into the uterus and then...