Illustration of stem cells known as spermatogonia following the first mitotic cell division of spermatogenesis (sperm cell production). This occurs within the seminiferous tubules in the testes (testicles). The upper right stem cell has divided to form another stem cell (top left) and a primary spermatocyte (centre). Spermatocytes then divide via meiosis to form spermatids, which in turn mature into spermatozoa, commonly known as 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.