may very probably be overlooked, or possibly be regarded as extraneous substances. But they are in fact the primitive structure of nerve, for the younger the foetus the greater is their relative quantity, and in a pig’s foetus of three inches in length, I found them the sole constituent of nerve, none of the fibres furnished with the dark margins, nor any of the cylinders or globules being visible at that period of development. The development of nerve, however, does not appear to proceed uniformly in all individuals; for the dark globules and cylinders were already present in some other pigs’ em- bryos, which were scarcely any larger. Pl. IV, fig. 6, represents a portion of the ischiatic, and fig. 7, of the brachial nerve of such a foetus. We observe a palish, and very minutely-granulated cord, which, in consequence of certain longitudinal shadings, such as the delineation exhibits, presents the appearance of a coarse fibrous structure. Round or for the most part oval corpuscles, which are immediately recognised as cell-nuclei, and which sometimes also contain one or two nucleoli, are generally seen in the course of these shaded parts, throughout the entire thickness of the cord. Sometimes a fibre separates from such a cord, and stands out isolated, as at a in both the figures, and the nuclei are then seen to lie in the course of the fibres. A single fibre presents several nuclei in its course, as was also observed in secondary muscle-cells (see fig. 8, 5), but I have never remarked it in the cells of the fourth class, the fibre-cells. Although the (nervous) fibres cannot at this early period be distinctly perceived to be hollow, the wall not being distinguishable microscopically from the contents, yet we shall see that the progress of development renders it highly probable that they are so. If then these (nervous) fibres are so far analogous to the early condition of secondary muscle-cells, that they are hollow, and in various parts of their course contain nuclei, whose form shows them to be ordinary cell-nuclei, it is probable that they are generated in a similar manner to muscle; that is, that they are formed by the coalescence of primary cells, to which the nuclei, just noticed as present upon the fibres, have pertained; so that thus the nervous fibres would be secondary cells, cor- responding to the secondary muscle-cells, or primitive muscular fasciculi. The actual observation of the primary cells of nerve
