Mature sperm means sperm that are able to swim independently. When the immature sperm cells move from the ducts of the testis into the epididymis, they are completely immobile and unable to fertilize an egg cell. As they travel slowly through the epididymis as a result of rhythmic contractions, they undergo maturation and become capable of moving independently so that they may engage in swimming motionsmycontentbreak after ejaculation.