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. The sperm mature as they pass slowly through this tube. Magnification: x2500 when printed at 10 centimetres wide.