Fig. 2

Spermatogenesis of Crasiella diplura and the spermatozoa of C. diplura and Crasiella sp. (All figures refer to C. diplura except n, u, v; a–f, h–v are TEM images, g is a DIC image). a Testes: the gametes mature in a caudo-cephalic (white arrow) and centripetal direction (black arrow). b Nucleus/mitochondrion of a spermatid: the chromatin starts to condense and the mitochondrion is surrounding the nuclear envelope. c Spermatid showing the cytoplasm filled with large vesicles from which the basal region of the acrosome develops. d Basal regions of acrosomes: the large vesicles are piled up on top of each other. e The U-shaped connection between nucleus and flagellum in a late spermatid. f Sperm duct in longitudinal section. g A single spermatozoon as seen by differential interference contrast optics. h The more apical region of the acrosome of in longitudinal section: a twisted tube with an inner structure made up of piled discs. i The second region of the acrosome of in longitudinal section: a straight tube surrounded by a helical ridge. j The third region of the acrosome in longitudinal section: note the small radial fibres visible also in cross-section in q. k The more proximal region of the acrosome in longitudinal section formed by large, piled, globular vesicles surrounded by smaller globular vesicles. l Longitudinal section of the nuclear–mitochondrial region and the basal acrosome. m Tangential section of the curled plasma membrane of the flagellum. n Longitudinal section of connection between the acrosome and nuclear–mitochondrial region of Crasiella sp.: note the thin mitochondrion and the low and the high density of large and small globular vesicles, respectively. Note also the low density of the granular material surrounding both types of vesicles. o–r Cross-sections of the acrosome, ordered from distal to proximal. s Cross-section of nuclear-mitochondrial region. t Cross-section of the flagellum: note the prominent central sheath. u,v Cross-section of the proximal region of the acrosome and the nuclear–mitochondrial region of Crasiella sp. a acrosome, at acrosomal tube, cm circular muscles, f flagellum, fs final spermatid, hr helical ridge, lgv globular vesicles, m mitochondrion, n nucleus, pm plasma membrane, pd piled discs, rf radial fibres, sd sperm duct, sgv small globular vesicles. Scale bars a = 4 μm; b, d, e, h, i–s, u, v = 0.3 μm; c, f, g = 4 μm; t = 0.1 μm