Fig. 1

Euspermatozoa of Adelomelon ancilla. a Longitudinal section (LS) through acrosomal complex and anterior portion of nucleus. b LS of the acrosomal complex in the anterior region. Note the apical bleb. c LS of the acrosomal complex. Note the constriction of the posterior acrosomal invagination (starting point indicated by arrow). d–g Series of transverse sections (TS) from apical bleb region (d) to basal region (g) of the acrosomal vesicle. Note lateral flattening of the acrosomal complex beginning in anterior portion of the invagination (d–f) and the presence of the accessory membrane (g). h LS junction of nucleus (showing invagination and centriole/axoneme insertion) and anterior portion of midpiece. i LS of midpiece. Note helical mitochondrial elements (defined by dense U-shaped profiles of periphery). j TS of the nucleus in the middle region. k TS of the nucleus in posterior region, note centriolar fossa with axoneme. l TS of midpiece region, note the microtubular pattern (9 + 2). A acrosomal complex, ab apical bleb, ar axial rod material, Av acrosomal vesicle, am accessory membrane, ax axoneme, bp basal plate, m mitochondrion, mp midpiece, N nucleus, sm subacrosomal material, U U-shaped defining edge of mitochondrial element