NERVOUS FIBRES. 145 
in their independent state, is very difficult, from the circum- 
stance of our being unable at that period to distinguish between 
them and the surrounding tissues ; for a whole organ is then 
composed entirely of independent cells, which have not as yet 
undergone any transformation. It is true I saw an independent 
cell, furnished with a nucleus, which seemed to have separated 
from the nervous cord, in one of the preparations alluded to, 
fig. 6 6; but I cannot positively assert that it had actually 
separated from that particular part, nor that it was a primary 
nerve-cell, for the cells in that preparation had not as yet un- 
dergone any change. In this instance, therefore, we must 
content ourselves, for the present at least, with the analogy to 
muscle. 
These fibres, or secondary nerve-cells, differ very much in 
their appearance from the subsequent nervous fibres, which are 
furnished with distinct but not dark outlines; they have a 
pale, granulated aspect. By progressive development, however, 
they become converted into the white fibres, and pl. IV, 
fig. 8, d, represents the transition. The part of the figure 
to the right hand exhibits the fibre yet in the early condition, 
pale, granulated, and furnished with a cell-nucleus ; in the 
portion to the left, it has completely assumed its subsequent 
form: it has a dark outline, is not granulated, and the one 
portion passes immediately into the other. The identity be- 
tween these pale fibres and the subsequent white nervous fibres 
is thus established. 
In what then does this transformation of the pale granu- 
lated fibres into the white fibres consist? Clearly in the 
development of the white substance ; we may, however, imagine 
three different modes in which this development may take 
place. It may take place, Istly. By the white substance form- 
ing as a sheath (cortex), around each fibre, and in this manner 
enclosing it. By this mode of explanation the fibre would be 
identical with the pale band discovered by Remak, which 
would therefore be the cell-membrane itself. 2dly. The white 
substance might be regarded as a transformation and thicken- 
ing of the cell-membrane of those fibres, or secondary nerve- 
cells. According to this view, the white substance would be 
the cell-membrane, and Remak’s band the firm contents of the 
secondary cell. 3dly. The white substance may be formed as 
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