Illustration showing semen moving through the urethra, a hollow tube in the penis, during ejaculation. Semen contains a mixture of sperm cells (spermatozoa, white) and secreted fluid from the seminal ducts, prostate and bulbourethral glands. The spermatozoa are the male gametes. They contain one set of genetic material, and can fertilise the female gamete (oocyte) during sexual reproduction. The seminal fluid contains enzymes and nutrients, and helps to provide energy and added motility to the sperm.