Figures 335 (upper) and 336 (lower) from Chapter 14 (Sperm Flagellum) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. Upper, cross-section of guinea pig sperm tail at the level of the midpiece; lower, cross-section at the level of the principal piece. Both samples were prepared using aldehyde fixation in the presence of tannic acid. When followed with osmium and uranyl acetate staining, this procedure improves contrast but may result in thickening of the doublets, arms and spokes. Upper image by Daniel Friend; lower from David Phillips. A PDF copy of the accompanying chapter is available on the ASCB’s BioEDUCATE website.