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 heads and tails (flagella) of nearly mature sperm are held in columns of Sertoli cell cytoplasm. In this species (guinea pig) the sperm heads are characteristically curved. The non-dividing cells, the Sertoli cells, extend from the base of the epithelium to the tubule lumen and provide structural and functional support for the maturation of the germ cells. Magnification x400 when printed at 10 cm.