Diagrammatic view of the stages of spermatogenesis along a seminiferous tubule. Stages, as Roman numerals, contain well defined and successive different associations of germ cells of the dynamic seminiferous epithelium. A complete sequence of stages is described as stages I-XIV, but at certain points along the tubule stages exhibit a reversal of the sequence indicated by arrows. These reverse segments are regulated by local mechanisms attributed to the Sertoli cells the only non-germinal cells in the seminiferous epithelium.