Mature spermatozoon of Collyricloides massanae. (a) Sharp morphology of the anterior spermatozoon extremity (ASE). (b, c) Cross-sections in Region I showing two centrioles (C1 and C2) corresponding to both axonemes and continuous layer of submembranous cortical microtubules (CM). Ax1, first axoneme. (d) Cross-section in which both axonemes are already formed and surrounded by a continuous layer of parallel cortical microtubules. (e, f) Consecutive cross-sections in the middle part of Region I exhibiting two and four attachment zones (arrows), interrupting the continuous layer of cortical microtubules. (g–i) Posterior part of Region I showing in cross-sections both axonemes and cortical microtubules organised into two fields separated by the four attachment zones. The appearance of the mitochondrion is also noticeable (M). Scale in μm: (a), 0.5; (b–i), 0.3.