The early days of electron microscopy provided the first views of cell ultrastructure. This electron micrograph shows the plasma membranes of two closely apposed glial cells from the central nervous system of the annelid Aphrodite (commonly known as a Sea mouse) obtained from a thin section. EM images of cell membranes, such as this one, suggested a tri-laminar structure to the membrane, detected as two thin dark lines separated by a layer of lighter intensity. The ability to visualize previously unseen ultrastructural elements of the cell, like cell membranes, fueled debate about their composition, organization, and biological function. The distance of the intercellular cleft between the two membranes is 15nm. This is Figure 1 from Chapter 1 (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.