146. We have seen good reason to conclude that the essential element of the nerve--the axis cylinder--is the same substance as the neuroplasm which forms the essential element of the cell. At any rate, we are quite certain that the cell process is neuroplasm. On this ground there is no difficulty in understanding that a cell process may sometimes be drawn out into an axis cylinder (as indeed we see to be the case in the invertebrata and electric fishes); while again in numerous other cases the nerve-fibre has an independent origin, being, in short, a differentiation from the neuroplasm which has become a fibre instead of a cell. It is clear from the observations of Rouget on Development, and of Sigmund Mayer on Regeneration, that fibres, nuclei, and cells become differentiated from the same neuroplasm, those portions which are not converted into fibres remaining first as lumps of neuroplasm, then acquiring a nucleus, and some of these passing into cells. I mean that between fibres, nuclei, and cells there are only morphological differences in an identical neuroplasm.[177] If this is in any degree true, it will not only explain how fresh fibres may be developed in the course of fibres, branching from them as from trunks, and branchlets from branchlets, twigs from branchlets, the same conditions of growth being present throughout; it will also completely modify the notion of any physiological distinction between cell and fibre greater than can be assigned to the morphological differences. We shall then no longer suppose that the cell is the fountain whence the fibre draws its nutrition and its “force”; and this will be equally the case even if we admit that a cell is, so to speak, the germ from which a whole plexus of fibres was evolved, for no one will pretend that the “force” of an organism is directly derived from the ovum, or that the ovum nourishes the organism.