Fig. 4

The spermatozoon of Diuronotus aspetos. a Schematic drawing of the filiform spermatic cell. b, c The two elongated cones forming the acrosome are visible (arrows): the larger one is very electron-dense and uniform in aspect, whereas the smaller one shows several microfilaments. d Apical portion of the small cone. e Detail of the insertion of the two acrosomial cones on the nucleus. The larger cone starts just at the nuclear apex, whereas the smaller one slightly behind. f, g The two nuclear regions are shown: the basal region is almost completely surrounded by a single, giant mitochondrion, the apical one is slightly larger and tapers only at the top (asterisk). h Oblique sections of the apical portion of the nucleus. i Cross section of the basal nuclear region. Note that the mitochondrion does not completely surround the nucleus, leaving a narrow space (arrow). j The flagellar axoneme sinks into a deep depression of the nuclear base, and it is surrounded by an outstanding system of nine accessory fibres. k Oblique section of the piece connecting head and tail. l A slightly oblique section of the flagellum. Note the moderately electron-dense contents (arrow) into the areas toward the doublets. m Longitudinal section of the flagellum: the electron-dense contents of the accessory fibres and their thin oblique striation (arrow) are clearly visible. n Cross section of the posterior region of the flagellum where the electron-dense contents of the accessory fibres disappears. d Depression, f flagellum, lc large acrosomal cone, m mitochondrion, sc small acrosomal cone. Scale bars b, d, e–h, j, k, l = 0.5 μm; c, i, m, n = 0.25 μm