THE SPINAL NEEVES IN ELASMOBEANCH PISHES. 
187 
possesses a covering of white matter ; this is thickest at the ventral portion of the 
cord, and extends to the region of the posterior root of the spinal nerve. 
The junction of the posterior root with the spinal cord is easier to observe than in the 
last stage. 
It is still effected by means of unaltered cells, though the cells which form the pro- 
jection from the cord to the nerve are commencing to undergo changes similar to those 
of the cells which are being converted into white matter. 
In the rudiment of the posterior root itself there are still three distinct parts, 
though their arrangement has undergone some alteration and their distinctness has 
become more marked (Plate 17. fig. 1 1 ). 
The root of the nerve (fig. 1 1, pr) consists, as before, of nearly circular cells, each 
containing a nucleus, very large in proportion to the size of the cell. The cells 
have a diameter of about 3 oroo °f an inch. This mass forms not only the junction 
between the ganglion and the spinal canal, but is also continued into a layer 
investing the outer side of the ganglion and continuous with the nerve beyond the 
ganglion. 
The cells which compose the ganglion (fig. 1 1, sp.g) are easily distinguished from 
those of the root. Each cell is elongated with an oval nucleus, large in proportion to 
the cell ; and its protoplasm appears to be continued into an angular, not to say fibrous 
process, sometimes at one and more rarely at both ends. The processes of the cells are 
at this stage very difficult to observe : figs. la, I b, I c represent three cells provided 
with them and placed in the positions they occupied in the ganglion. 
The relatively very small amount of protoplasm in comparison to the nucleus is fairly 
represented in these figures, though not in the drawing of the ganglion as a whole. In 
the centre of each nucleus is a nucleolus. 
Fig. I b, in which the process points towards the root of the nerve, I regard as a com- 
mencing nerve-fibre : its more elongated shape seems to imply this. In the next stage 
special bundles of nerve-fibres become very conspicuous in the ganglion. The long 
diameter of an average ganglion-cell is about °f an inch. The whole ganglion 
forms an oval mass, well separated both from the nerve-root and the nerve, and is not 
markedly continuous with either. On its outer side lies the downward process of the 
nerve-root before mentioned. 
The nerve itself is still, as in the last case, composed of cells which are larger and 
more elongated than either the cells of the root or the ganglion. 
The condition of the anterior root at this stage is hardly altered from what it was ; 
it is composed of very small cells, which with haematoxylin stain more deeply than any 
other cell of the section. A figure of it is given in I n. 
Horizontal longitudinal sections of this stage are both easy to make and very instruc- 
tive. On Plate 18. fig. K i is represented a horizontal section through a plane near the 
dorsal surface of the spinal cord : each posterior root is seen in this section to lie nearly 
opposite the anterior extremity of a muscle-plate. 
