Epididymis. Coloured scanning electron micrograph (SEM) of a freeze-fractured plane through the epididymis. Aggregates of healthy spermatozoa (sperm, pink) fill this tube, a part of the male reproductive system. The epididymis is a 7-metre- long coiled tube that lies behind each testis, receiving sperm from them. The sperm mature as they pass slowly through this tube. Magnification unknown.