(Plural: spermatozoa.) This is the male sex or germ cell which unites with the OVUM to form the EMBRYO or fetus. It is a highly mobile cell approximately 4 micrometres in length – much smaller than an ovum, which is about 35 micrometres in diameter. Each millilitre of SEMEN containing about 100 million spermatozoa.

Once ejaculated during intercourse the spermatozoon travels at a rate of 1·5–3 millimetres a minute and remains mobile for several days after insemination, but quickly loses its potency for fertilisation. As it takes only about 70 minutes to reach the ovarian end of the uterine tube, it is assumed that there must be factors other than its own mobility, such as contraction of the muscle of the womb and uterine tube, that speed it on its way.