This electron micrograph shows a cross section through the midpiece of opossum sperm. Filamentous cytoplasm underlies a scalloped membrane with regular variations in thickness, which gives it a corrugated appearance. Aat the center is an arrangement of dense fibers and microtubule doublets. Figure from page 20, Chapter 1 (The Cell 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.