Light microscopy of a seminiferous tubule in the testis showing full normal spermatogenesis. The immature germ cells are at the margin of the tubule and progressively more mature germ cells form layers towards the central lumen which contains fluid. The long tails (flagella) of nearly mature sperm are held in columns of Sertoli cell cytoplasm. Sertoli cell nuclei with a single dense nucleolus are found at the base of the epithelium. Magnification x320 when printed at 10 cm.