144 NERVOUS FIBRES. 
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 deve- 
lopment. 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, repre- 
sents 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, pre- 
sents the appearance of a coarse fibrous structure. Round or 
for the most part oval corpuscles, which are immediately recog- 
nised 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, 4), 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 gene- 
rated 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 pertaimed; 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 
