Light microscopy of a seminiferous tubule in the testis. Note the orderly arrangement of germ cells of the seminiferous epithelium. Starting from the periphery, successive generations of germ cells multiply and mature in a highly coordinated, synchronous patterns as evidenced by many spermatids (rounded, pale nuclei) all at the same stage of development. More advanced germ cells are seen as sperm, entering the fluid-filled lumen. The coordination of spermatogenesis is species-specific and of fixed duration for example in man about 70 days, rat 50 days, mouse 35 days. Control of germ cell development is exerted by Sertoli cells which lie at regular intervals along the base of the seminiferous epithelium. Magnification x250 when printed at 10 cm.