Illustration of sperm cells (spermatozoa) moving through the epididymis. Spermatozoa mature as they travel from the testes (testicles) to the epididymis, a structure behind the testes. Here they are stored until sexual stimulation, at which point they are transported to the vas deferens (sperm duct). Spermatozoa are reproductive cells (gametes) which can fertilise the egg during sexual reproduction, and carry one set of chromosomes from one parent.