Phase contrast microscopy of living cells offers the ability to observe dynamic behaviors. This single frame image shows folds in the membrane at the periphery of a cell in growing in vitro. Image by Charles Pomerat, and published in Fawcett (1964) J. Histochem. Cytochem. 13:75-91, reprinted with permission as Figure 47 from Chapter 2 (Specializations of the Free Surface) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.