Mature spermatozoon of Collyricloides massanae. (a) Middle part of Region IV characterised by the disorganisation of the first axoneme into doublets (D). M, mitochondrion; N, nucleus. (b) Cross-section showing one axoneme, mitochondrion (M), nucleus (N) and cortical microtubules (CM). (c) Cross-section after mitochondrion disappearance exhibiting only nucleus (N), axoneme and few cortical microtubules (CM). (d, e) Consecutive cross-sections in the distal part of Region IV showing progressive decrease of nucleus diameter and appearance of “cytoplasmic stalk” (CS) between the nucleus (N) and the axoneme (Ax). Note also the disappearance of cortical microtubules. (f) Posterior spermatozoon tip with doublet (D) of microtubules. (g) Cross-sections showing the granules of glycogen (G) evidenced by Thiéry’s cytochemical test [51]. Scale in μm: 0.3.