aspect, the inner surface of which constantly exhibits cell- nuclei in the very early period of the development of nerve ; but in the somewhat more advanced stage, when the white substance is developed, they are only occasionally found. 2dly. That the white, fat-like substance to which the peculiar appearance and distinct outline of the nerves are chiefly referable, is deposited upon the inner surface of this cell-membrane. When this deposit is thick, its double contour (to which the nerve is indebted for its tubular appearance), may be recognised; this, however, escapes observation when only a thin stratum of white substance is present. Morphologically considered, it therefore corresponds to the peculiar substance of muscle, for that is likewise developed as a secondary deposit upon the membrane of the secondary muscle-cell. 3dly. That the rest of the cell-cavity appears to be filled up by a firm substance, namely, the band discovered by Remak. There seems to be no structure analogous to this band in perfectly-developed muscles, for there, the secondary deposit, that is, the formation of the proper muscular substance, proceeds until the cavity of the secondary cell is completely filled.
We have thus traced the development of nerve to its perfect state, without those irregular globules and little cylinders with the dark outlines, (which were mentioned at page 143, as occurring at a middle stage of the development of nerve in addition to the pale fibres and the matured nervous fibres), having proved to be a transitional step in the process. I am inclined to regard them as an artificial product, caused by pressure and the action of water upon the as yet very delicate nerve. If, for example, water penetrate through the cell-membrane by imbibition, the oil-like white substance retracts into separate rounded bodies, and the facility with which this takes place is proportionate to its slight degree of consistence. This is often seen even in fully-developed nerves; an entire nerve frequently separates from this cause into separate globules or little cylinders, which have sharply-defined outlines, so that merely the cell-membrane proceeds uninter- ruptedly, in the form of a pale stripe, from the external wall of one of the dark portions to that of the other. Valentin has given a delineation of such a state of the nervous fibre, (Acta Acad. Leopold. Nat. Curios. vol. xviii, pl. III, fig. 7). As the
