In Cerastoderma glaucum, Sertoli cells are rich in lipids, glycogen and lysosomes, and premeiotic cells exhibited nuage, a prominent Golgi complex and endoplasmic reticulum. cisternae encircling the nucleus. The Golgi complex gives rise to proacrosomal vesicles during mid-spermiogenesis, and the round acrosomal vesicle, with a dense fibrillar core, migrates laterally while linked to the plasma membrane as it develops the subacrosomal material. In its final position, the vesicle becomes cap-shaped (0.6 mum) and differentiates into apical light and basal dense regions. The elongated and helicoidal nucleus (8-9.9 mum) has a thin tip (0.3 mum) that invades the subacrosomal space, and in the midpiece (0.8 mum) two of the four mitochondria extend laterally to the nucleus (1.5-2.1 mum). In Spisula subtruncata, Sertoli cells are rich in lipids, glycogen and phagocytosed sperm. Premeiotic cells exhibit nuage, a prominent Golgi complex that gives rise to proacrosomal vesicles from the leptotene stage and a flagellum that is extruded at zygotene. The acrosomal vesicle forms during the round spermatid stage and differentiates into a large and dense basal region and an apical light region. It then migrates while linked to the plasma membrane by its apical pole. Development of the subacrosomal perforatorium is associated with nuage materials and endoplasmic reticulum. vesicles. The mature cap-shaped (0.6 mum) acrosomal vesicle exhibits a large apical and irregular region with floccular contents and a basal dense region. The round nucleus becomes barrel-shaped (1.5 mum) and the the midpiece (0.8 mum), with four mitochondria, contains a few glycogen particles.