testis (tĕsˈtĭs) [key] or testicle tĕsˈtĭkəl, one of a pair of glands that produce the male reproductive cells, or sperm. In fetal life the testes develop in the abdomen, then descend into an external sac, the scrotum. A testis is composed of about 800 coiled seminiferous tubules whose linings contain cells that develop into sperm. These tubules merge into a larger tube called the epididymis that leads out of the testis into the vas deferens. Between the seminiferous tubules there is interstitial tissue that secretes the male sex hormone testosterone, which stimulates the development of the male reproductive system and secondary sex characteristics. Sometimes the testes do not descend normally before or shortly after birth and remain in the abdomen or groin. Such a condition requires medical attention or the gland will eventually become sterile. See also reproductive system.