It is a narrow, tightly-coiled tube connecting the efferent ducts from the rear of each testicle to its vas deferens
. The epididymis can be divided into three main regions, the head (caput), body (corpus) and tail (cauda). Sperm 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.