The development and structure of myelin sheaths have been studied in the optic nerves of rats and of Xenopus laevis tadpoles. Both potassium permanganate- and osmium-fixed material was examined with the electron microscope. In the first stage of myelinogenesis the nerve fibre is surrounded by a cell process which envelops it and forms a mesaxon. The mesaxon then elongates into a loose spiral from which the cytoplasm is later excluded, so that compact myelin is formed. This process is similar to myelinogenesis in the peripheral nervous system, although in central fibres the cytoplasm on the outside of the myelin is confined in a tongue-like process to a fraction of the circumference, leaving the remainder of the sheath uncovered, so that contacts are possible between adjacent myelin sheaths. The structure of nodes in the central nervous system has been described and it is suggested that the oligodendrocytes may be the myelin-forming cells.

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