A–I. Mature spermatozoon of Atractotrema sigani in regions I–III. (A) Longitudinal section of anterior spermatozoon extremity (Ase) with a sharp tip. Sdm, submembranous electron-dense material; Ax1 first axoneme. (B) Cross-section showing the anterior extremity of the first axoneme (Aae1) and the submembranous electron-dense material describing a complete ring. (C) Cross-section in region I exhibiting the first axoneme and the anterior extremity of the second axoneme (Aae2). Note also the submembranous electron-dense material surrounding both axonemes. (D) Detail of submembranous electron-dense material of A. sigani. Pm, plasma membrane; Sdm, submembranous electron-dense material. (E) Cross-section in posterior part of region I with the axonemes and the submembranous electron-dense material located laterally. (F, G) Consecutive cross-sections in region II characterized by the presence of external ornamentation of the plasma membrane (Eo), the first mitochondrion (M1) and spine-like body (Sb). Note the cortical microtubules (Cm) associated with the external ornamentation and the residual submembranous electron-dense material not associated with cortical microtubules. (H) Detail showing the simultaneous presence of external ornamentation, spine-like body and submembranous electron-dense material. (I) Cross-section in the transitional area prior to the nuclear region, showing the axonemes and cortical microtubules distributed into two fields and their maximum number varies between 14 and 17. Scale in μm: (A, B, C, E, F, G, I), 0.3; (D), 0.2; (H), 0.1.