The reproductive organ in male animals in which spermatozoa are produced. In vertebrates there are two testes; as well as sperm, they produce steroid hormones (see androgen). In most animals the testes are within the body cavity but in mammals, although they develop within the body near the kidneys, they come to hang outside the body cavity in a scrotum. Most of the vertebrate testis is made up of a mass of seminiferous tubules, lined with Sertoli cells, in which the sperms develop (see spermatogenesis). It is connected to the outside by means of the vas deferens. See reproductive system.