For the past several years we have conducted studies on the morphology of myelinated fibers in the central nervous system and in the neurogenic electrocytes of the gymnotid Sternarchus albifrons. These investigations have demonstrated a high degree of regional differentiation in both the central nervous system and in the more $ accessible electrocyte axons. There is at present a paucity of information about the mechanisms by which regional differentiation and specificity in myelination are determined. Such information may be important to an understanding of reparative mechanisms in the demyelinating diseases. The objective of the proposed research is a fuller delineation of the regional morphology of myelinated fibers, and of the developmental mechanisms by which specificity in myelination is achieved. The first phase of the proposed project will be a histochemical and ultrastructural study on fully developed and regenerating and remyelinating electrocyte axons, with special reference to the changes in the axon-Schwann cell relationship which occur along the course of these fibers, and to the question of dependence of motor neurons on perpheral connections. The second phase of the project will be devoted to extending the dimensional analysis of central myelinated fibers to the rhesus monkey. Particular attention will be given to the dimensional properties of preterminal fibers in the neuropil; previously available data concerning these fibers is limited to inframammalian species. In the third phase of the proposed project, we will study the morphology of serial internodes from each of a population of contiguous myelinated fibers, with the goal of characterizing the population of fibers and myelinated internodes associated with a single glial element.