The body of the epididymis (plural, epididymides) is a tightly coiled, thread-like tube that is about six meters long. This tube is connected to ducts inside the testis. The head emerges from the top of the testis, descends along the outer surface, and then moves upward to become the vas deferens at its tail. The inner lining of the epididymis is made of columnar cells that are thought to secrete glycogen, which helps keep stored sperm cells alive. 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 motions after ejaculation.