TEM micrographs of S. yangi sperm head region. A Longitudinal section of sperm head, showing the head region (including acrosome (a) and nucleus (n)) inserted into a homogenous matrix (ma) B Cross-section through the tip of acrosome (a), showing acrosome is surrounded by a homogenous matrix (ma) C Cross-section through the mid-acrosome (a), showing acrosome (a) and subacrosomal space (ss) D and E Cross-sections of base of acrosome (a), showing nucleus (n) and two acrosomal processes F Cross-section through circular nucleus (n). Scale bars: 500 nm (A), 200 nm (B–F).