THE SPINAL COED IN MAN, MAMMALIA, AND BIEDS. 
925 
they were enveloped in delicate sheaths, which, however, were quite distinct from, 
although in connexion with, the surrounding reticular tissue (see figs. 12 & 15, Plate 
XLVn.). Many of then: processes also were very clearly seen to subdivide, or to break 
up suddenly, into a multitude of fine branches to form part of the intervening net- 
work f. 
I shall conclude my remarks on the development of the cord in the human and 
mammalian foetus, by a few observations on the development of its nerve-fibres. In 
a very young foetus it is difficult to obtain a satisfactory view of isolated nerve-fibres, 
and to detect the way in which their formation commences. According to my own 
opportunities of observation, they are not developed from nucleated cells, but rather 
by the extension of finely granular substance from round and oval nuclei. On this 
point, however, I cannot at present speak with confidence, and therefore leave it open 
for further inquiry. In the early stages — for instance, in a foetal sheep, or human 
foetus from 1 to 2 inches long — the fibres, in a fresh state, consisted of most delicately 
granular and nucleated bands, without any sharpness of outline or appearance of sepa- 
rate border. But the nuclei were far from being numerous, either in the nerves or in 
the white columns of the cord. In fig. 8, Plate XLVI. their average number is shown 
in the white columns of the left side, in a human foetus of about nine weeks. As 
development, however, advances, their number increases considerably, while the fibres 
to which they belong acquire a more sharply defined outline or border, which in some 
parts of its course appears darker and thicker than in others. In fig. O’*", Plate XLV.,* 
on the left, is an exact representation of a separate fibre in a fresh and unprepared 
state, from the sciatic nerve of a human foetus of four months, magnified 670 diameters ; 
and in the same figure, on the right, is represented the appearance of several such fibres 
as they lie side by side in a bundle. Fig. 18, Plate XLVII. shows a small portion of a 
transverse section of the posterior white columns of a human foetus of five months, 
magnified 670 diameters. When compared with the same parts in fig. 8, Plate XLVI., 
which is magnified only 50 diameters, it shows how much the nuclei have increased in 
number. Some of these nuclei belong to the sheaths of the nerve-fibres, others to the 
tissue by which their sheaths are connected. As the period of birth approaches, they 
are again reduced in number ; but even in the adult cord, as I showed on a former 
occasion, they are scattered at intervals between the fibres of all the white columns. 
In structure they are altogether similar to the nuclei of the foetal epithelium above 
described, and differ from them only in having a somewhat less average diameter. 
With regard to the development of the spinal cord in Birds, I need dwell only on 
those particulars in which it differs from that in Man and Mammals. Fig. 19, 
Plate XLVII. shows the appearance of the grey and white substance in a transverse 
t I faye described and figured a similar ramification of the processes of the cells around the base of the 
peduncle of the olfactory bulb, and in the anterior perforated space, and have frequently observed the same 
appearance in the convolutions of the cerebral hemispheres. (See Zeitschrift fur 'Wissensch. Zoologie, 
Ed. xi. Hft. 1. Taf. v. fig. 6.) 
