THE SPINAL NERVES IN ELASMOBRANCH PISHES. 
181 
From its ventral end is continued the nerve, which is of considerable length, and has 
a course approximately parallel to that of the muscle-plate. It forms a continuation of 
the root rather than of the ganglion. 
Further details in reference to the histology of the nerve-rudiment at this stage are 
given later in this paper, in the description of Pristiurus-e mbryos, of which I have a 
more complete series of sections than of the Torpedo-e mbryos. 
When compared with the nerve-rudiment in the posterior part of the same embryo, 
the nerve-rudiment last described is, in the first place, considerably larger, and has 
secondly undergone changes, so that it is possible to recognize in it parts which can be 
histologically distinguished as nerve and ganglion. 
The developmental changes which have taken place in the anterior root are not less 
important than those in the posterior. The anterior root now forms a very conspicuous 
cellular prominence growing out from the ventral corner of the spinal cord (fig. D c, ar). 
It has a straight course from the spinal cord to the muscle-plate, and there shows a 
tendency to turn downwards at an open angle : this, however, is not represented in the 
specimen figured. The cells of which it is composed each contain a large oval nucleus, 
and are not unlike the cells which form the posterior rudiment. The anterior and 
posterior nerves are still quite unconnected with each other ; and in those sections in 
which the anterior root is present the posterior root of the same side is either com- 
pletely absent or only a small part is to be seen. The cells of the spinal cord exhibit 
a slight tendency to converge towards the origin of the anterior nerve-root. 
In the spinal cord itself the epithelium of the central canal is commencing to become 
distinguished from the grey matter, hut no trace of the white matter is visible. 
I have succeeded in making longitudinal vertical sections of this stage, which prove 
that the ends of the posterior roots adjoining the junction with the cord are all connected 
with each other (Plate 16. fig. D d). 
If the figure representing a transverse section of the embryo (fig.D c) be examined, 
or better still the figure of a section of the slightly older Scy Ilium-embryo (Plate 17. 
fig. H i or 1 1), the posterior root will be seen to end dorsally in a rounded extremity, and 
the junction with the spinal cord to be effected, not by the extremity of the nerve, but 
by a part of it at some little distance from this. 
It is from these upper ends of the rudiments beyond the junction with the spinal cord 
that I believe the commissures to spring which connect together the posterior roots. 
My sections showing this for the stage under consideration are not quite as satisfactory 
as is desirable ; nevertheless they are sufficiently good to remove all doubt as to the 
presence of these commissures. 
A figure of one of these sections is represented (Plate 16. fig. Dd). In this figure 
pr points to the posterior roots and x to the commissures uniting them. 
In a stage somewhat subsequent to this I have succeeded in making longitudinal 
sections, which exhibit these junctions with a clearness which leaves nothing to be 
desired. 
