RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY
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Testis. Illustration of a single human testis. The testis is the site of sperm formation and also produces androgen, a male sex hormone. The main part of the testis (centre) contains many seminiferous tubules (not seen) where sperm production (spermatogenesis) occurs. These tubules are arranged in lobes that radiate from a central plexus. Sperm then pass to the epididymis (at left and upper centre), a single convoluted tube where they are stored while they mature. During ejaculation, sperm enter the vas deferens duct (upper left) and are ejected out of the penis through the urethra.
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