Scanning electron micrograph showing a lateral view of sperm penetration of a hamster oocyte during vitro fertilization. As the membrane of the sperm head fuses with that of the oocyte, the microvilli on the oocyte reform over the hybrid membrane. Image by David Phillips, Figure 33 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.