Figures 399 (upper) and 400 (lower) from Chapter 16 (Cytoplasmic matrix and cytoskeleton) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. Cytoplasmic microtubules, shown in cross section. The upper panel shows a common arrangement of 13 protofilaments; the lower panel is a preparation of microtubules assembled in vitro from flagellar tubulin, composed of 15 protofilaments. Upper image contributed by Vitauts Kalnins; lower image from Binder and Rosenbaum, J. Cell Biol. 79:500-515, 1978 (PMID:569158). A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.