a simple cell, and since the simple cell-membrane continues to exist distinct from its secondary deposits, and from the cell-contents, it is quite conceivable that fibres may be generated in the secondary deposits or in the cell-contents, as they are in muscle, although we have as yet no evidence of the fact; but these fibres could no more issue out free from the white nervous fibre, than the primitive fibres of muscle could from secondary muscle-cell, because, in order to do so, they must necessarily rupture the cell-membrane of the secondary cell. These subdivisions, therefore, so far as the structure from whence they issue corresponds to an ordinary nervous fibre, and is not merely a fasciculus of very minute secondary nerve-cells, cannot be a mere appearance, nor anything but actual divisions, a simple secondary nerve-cell becoming eiongated into several minute fibres, in a manner analogous to that which we have witnessed in the fibre-cells, (see page 115.) The nerves in the tail of the tadpole may therefore be described as terminating by the nervous fibres, that is, the secondary cells becoming split in different directions after the manner of fibre-cells or stellate cells. In the memoir before alluded to, I have noticed some swellings upon the pale nervous fibres in the tail of the tadpole. They have a double signification; some which are marked off from the rest of the fibre by a sharply-defined outline are the nuclei of the cells, from which the fibres have been generated; the majority, however, which pass into the fibre without a well-defined contour, as generally occurs at situations where the fibres divide and diverge towards different sides, are the bodies of the original cells, which (especially when they become elongated at different parts into fibres) remain somewhat thicker than the prolongations themselves; the pigment-cells, pl. II, fig. 9 a, exhibit this appearance.
b. Gray or organic nervous fibres. The gray cords, which, according to the researches of Retzius and J. Müller, are derived from the sympathetic nervous system, and mingled with the cerebrospinal nerves in which they sometimes pursue a long isolated course, owe their gray appearance, according to the investigations of Remak, “to the peculiar structure of the primi- tive fibres, which arise in the ganglia. They are not tubular,
