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The epididymis is part of the male reproductive system and is present in all male amniotes. It is a single, narrow, tightly-coiled tube (in adult humans, six to seven meters in length) connecting the efferent ducts from the rear of each testicle to its vas deferens. A similar, but probably non-homologous, structure is found in cartilaginous fishes. The epididymis can be divided into three main regions: The head (Caput). The head of the epididymis receives spermatozoa via the efferent ducts of the mediastinum of the testis. It is characterized histologically by a thin myoepithelium. The concentration of the sperm here is dilute. The body (Corpus) The tail (Cauda). This has a thicker myoepithelium than the head region, as it is involved in absorbing fluid to make the sperm more concentrated. Role in storage of sperm and ejaculant Spermatozoa formed in the testis enter the caput epididymis, progress to the corpus, and finally reach the cauda region, where they are stored. Sperm entering the caput epididymis are incomplete - they lack the ability to swim forward (motility) and to fertilize an egg. It stores the sperm for 2–3 months. During their transit in the epididymis, sperm undergo maturation processes necessary for them to acquire these functions. Final maturation is completed in the female reproductive tract (capacitation). During ejaculation, sperm flow from the lower portion of the epididymis (which functions as a storage reservoir). They have not been activated by products from the prostate gland, and they are unable to swim, but are transported via the peristaltic action of muscle layers within the vas deferens, and are mixed with the diluting fluids of the seminal vesicles and other accessory glands prior to ejaculation (forming semen). The epithelial cells of the epididymis possess numerous apical modifications that are often referred to as stereocilia, as under the light microscope they look like cilia. However, as electron microscopy has revealed them to be structurally and functionally more similar to microvilli, some now refer to them as stereovilli.