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Testicle, or testis (plural: testes), one of paired male sex glands. The testes are contained in the scrotal sac just between the legs. They have two functions. As ductless glands, they secrete testosterone, the male sex hormone. This substance is responsible for maintenance of the accessory reproductive organs and also for the secondary sexual characteristics—deep voice, strong muscles, and facial hair. Their other function is to produce the sperm needed to fertilize the female egg. Sperm is produced in a series of long coiled tubes called seminiferous tubules. In higher mammals, this process requires a temperature lower than normal body temperature, a condition provided by having the scrotal sac outside the body cavity. Mature sperm are stored in a coiled tube, the epididymis, up to 20 ft (6 m) long, which leads from the rear of the testis to the ejaculatory duct.

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