An ultrastructural drawing of a transverse section through a myelinated nerve fiber. The axon (A) is surrounded by myelin and then by lemmocyte cytoplasm and lemmocyte nucleus (N), all within a basal lamina. The inset shows myelin at high magnification. Myelin is composed of multiple layers of plasma membrane, formed by neurolemmocyte processes that wrap many times around the axon and then extrude cytoplasm. The neurolemmocyte processes form an outer mesaxon (o.mes) at the surface. The inset shows that myelin exhibits major and minor dense lines, formed by merger of inner and outer membrane surfaces, respectively.