A figure from Chapter 13 (Cilia and Flagella) of 'The Cell' by Don W. Fawcett M.D. The microtubules of the axoneme are adequately preserved by routine fixation for electron microscopy of thin sections. These methods are less satisfactory for their subunits and appendages and have contributed little to our understanding of the organization of the ciliary matrix. Addition of tannic acid to the fixative results in an apparent thickening and a more intense staining of these components. The dynein arms are made more conspicuous, the radial spokes more robust, and the unstained protofilaments in the wall of the microtubules are clearly revealed in negative image. A copy of the chapter is available on the ASCB's BioEDUCATE website.