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Light micrograph of a vertical section through a human testis showing numerous seminiferous tubules, the site of production & maturation of sperm. The testis is packed with numerous tubules which are lined by a stratified epithelium consisting of 2 distinct groups of cells. One group of cells is the spermatogenic series, the cells involved in both spermatogenesis (production of male gametes) and subsequent spermiogenesis (evolution of male gametes into motile spermatozoa). The other group are non-spermatogenic cells called Sertoli cells, which nourish the developing spermatozoa. (KEYSTONE/SCIENCE SOURCE/BIOPHOTO ASSOC.)