Pig sperm. Coloured scanning electron micrograph (SEM) of immature pig sperm in the epididymis. This immature sperm is characterised by a small amount of unshed residual cytoplasm below the head. The epididymis is a 7-metre- long coiled tube that lies behind each testis, receiving sperm from them. Numerous characteristic stereocilia of the epididymis nourish the developing sperm. The sperm mature as they pass slowly through this tube. Magnification: x5000 when printed at 10 centimetres wide.