150 NERVOUS FIBRES. 
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 gene- 
rally 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 prolonga- 
tions 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. Miler, are derived 
from the sympathetic nervous system, and mingled with the 
cerebrospinal nerves 11 which they sometimes pursue a long 
isolated course, owe their gray appearance, according to the in- 
vestigations of Remak, “to the peculiar structure of the primi- 
tive fibres, which arise in the ganglia. They are not tubular, 
