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Cover  This month's cover illustration shows Spermiogenesis of the ratDrawing (prepared by Clermont and Rambourg, 1978) of spermiogenesis of the rat, as seen with the light microscope in sections stained with toluidine blue and periodic acid-Schiff. The numbers of the drawings correspond to the steps of spermiogenesis as proposed by Leblond and Clermont (1952). During the first three steps of spermiogenesis, the Golgi phase, the juxtanuclear Golgi apparatus elaborates proacrosomic granules, which fuse into a single acrosomic granule that attaches itself to the nucleus. During steps 4 to 7, or cap phase, the Golgi apparatus contributes to the formation of the acrosome that spreads on the surface of the nucleus. During this phase the mitochondria are along the plasma membrane, and the miniscule centrioles bind to the nucleus at the pole opposite the acrosome and give rise to the axoneme of the growing tail. During steps 8 to 14, the acrosome phase, the spermatid becomes polarized, the chromatin condenses, and the nucleus elongates to take its characteristic and elegant sickle shape. Simultaneously, the Golgi apparatus stops adding glycoproteins to the acrosome and migrates in the cytoplasmic lobe that extends along the forming tail. During steps 15 to 19, the maturation phase, the tail completes its formation as the mitochondria migrate towards the proximal segment of the developing tail to form the mitochondrial sheath. The nucleus completes its condensation and acquires a thin shell, the perinuclear theca or perforatorium, on which the acrosome binds. Finally, the cytoplasmic lobe flows apically, and the bulk of the unused cytoplasm separates from the spermatid that then becomes a spermatozoon. All these perfectly timed and integrated events have been analyzed with the electron microscope (reviewed by Clermont, Oko, and Hermo, 1993). Such studies underscore the complexity of these cytological events, which presuppose a multitude of regulatory mechanisms, biochemical and physiochemical in nature, most of which have yet to be clarified.by Yves Clermont