Mature spermatozoon of Bucephalus margaritae. (a)–(d) Cross-sections in region I of the spermatozoon showing the anterior extremity of the spermatozoon with only an external ornamentation of the plasma membrane, cortical microtubules under the plasma membrane and the lateral expansion (a). Then, the two axonemes appear progressively (b)–(c). In addition to the previous structures, the posterior extremity of this region exhibits undeveloped spine-like bodies (d). (e)–(i) Cross-section in region II of the mature spermatozoon showing only the two axonemes and cortical microtubules (e); disorganization of axoneme 1 (f); only one axoneme and cortical microtubules (g); appearance of the mitochondrion and the nucleus (h–i). (j)–(n) Cross-sections in the posterior region of the mature spermatozoon showing only the second axoneme, mitochondrion and nucleus (j), then disorganization of the axoneme (k), disappearance of the second axoneme (l), disappearance of the mitochondrion, and finally the posterior extremity of the mature spermatozoon with only the nucleus (m)–(n). Aae2 = anterior extremity of the second axoneme, Ax1 = first axoneme, Ax2 = second axoneme, Ax = axoneme, Cm = cortical microtubules, Eo = external ornamentation, Le = lateral expansion, M = mitochondrion, Mi = microtubule, N = Nucleus, Pae1 = posterior extremity of the first axoneme, Pae2 = posterior extremity of the second axoneme, Sb = spine-like body. Scale Bars = 0.2 μm.