Neurofibrils were discovered by Remak in 1843. They were subsequently studied by Max Schultze, who maintained that they were characteristic of living nerve cells and fibers and were the transmitting parts of these elements. Schultze may be looked upon as the founder of the neurofibril hypothesis. Much opposition developed to this view, and neurofibrils were regarded by many investigators as artifacts. Staunch supporters of Schultze''s hypothesis were found in Apathy and Bethe Within the last year or two Bozler has demonstrated neurofibrils in the living nerve cells of a jelly-fish. These observations, coupled with those of the earlier workers, place the existence of neurofibrils as constituents of living nerve cells beyond doubt. In function they have been regarded by one school of histologists as transmitting organs, and by another as supporting organs. On the basis of their distribution it is suggested in the present paper that neurofibrils serve for the conduction of metabolic influences, rather than of nerve impulses. The same view is applied to the fibrillar systems in ciliated cells and in certain ciliated protozoans.