Sperm. Coloured scanning electron micrograph (SEM) of mature sperm in a seminiferous tubule that has been freeze fractured. The fracture plane has cut through the heads (top left) of the sperm. Their middle sections run across upper left, and their tails occupy the lower right half of this image. The heads of the sperm form part of the wall of this seminiferous tubule, and the tails are in the central space of the tube. In human males, the seminiferous tubules are found in the testes. The developing sperm form in the outer layer of the tubule, maturing as they move inwards towards the centre, finally being fully motile (free-moving). Magnification unknown.