Abstracted from an image by Simionescu and Simionescu in The Journal of Cell Biology (70:608-621), ©1976, The Rockefeller University Press. This image was reprinted as Figure 319 from Chapter 13 (Cilia and Flagella) 'The Cell' by Don W. Fawcett M.D.(1981, W.B. Saunders Company). Ciliated cells of mammals often have microvilli interspersed among the cilia. These transverse sections near the cell surface permit a comparison of the size and internal structure of the two types of cell process. Both are limited by a trilaminar unit membrane and have a cytoplasmic matrix of similar density. The nonmotile microvilli have no resolvable internal structure, whereas the motile cilia have a consistent pattern of nine doublet microtubules uniformly spaced around a central pair of singlet microtubules. This nine plus two complex was visualized by the early light microscopists as a single axial fiber and was therefore called the axoneme. The term has been retained even though it does not accurately describe the microtubule complex revealed by the electron microscope. A copy of Chapter 13 from 'The Cell' is available on the ASCB's BioEDUCATE website.