Illustration of the start of sperm cell formation (spermatogenesis) within the seminiferous tubule of a testis (testicle). Stem cells called spermatogonia (clear cell top-centre) divide and differentiate to form either cells which produce more stem cells, or spermatocytes. Spermatocytes then divide via meiosis to form spermatids, which in turn mature into spermatozoa, commonly known as sperm. In spermatogenesis, stem cells called spermatogonia divide and differentiate, eventually forming spermatozoa (sperm). Spermatozoa are the mobile male gametes (reproductive cells), which can fertilise the egg during sexual reproduction, and carry one set of chromosomes from one parent.