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In male animals, each meiosis produces four equal-sized sperm cells in a process called spermatogenesis. In vertebrates, a cell type in the testes known as a spermatogonium produces primary spermatocytes, as well as additional spermatogonia, by mitosis. The primary spermatocytes then undergo meiosis. After the first meiotic division, these cells are known as secondary spermatocytes; after the second meiotic division, they are known as spermatids. The spermatids mature into spermatozoa by a process called spermiogenesis with four sperm cells resulting from each primary spermatocyte. In human beings and other vertebrates without a specific mating season, the process of spermatogenesis is continuous throughout adult life. A normal human male may produce several hundred million sperm cells per day.