In sections taken from the hinder part of the body I found that the posterior rudiments nearly agreed in size with those in fig. D b.
It is, however, still less easy than there to trace the junction of the posterior rudiments with the spinal cord, and the upper ends of the rudiments of the two sides do not nearly meet.
In a considerable series of sections I failed to find any case in which I could be absolutely certain that a junction between the nerve and the spinal cord was effected; and it is possible that in course of the change of position which this junction undergoes there may be for a short period a break of continuity between the nerve and the cord. This, however, I do not think probable. But if it takes place at all, it takes place before the nerve becomes functionally active, and so cannot be looked upon as possessing any physiological significance.
The rudiment of the posterior nerve in the hinder portion of the body is still approximately homogeneous, and no distinction of parts can be found in it.
In the same region of the body the anterior rudiment retains nearly the same condition as in the previous stage, though it has somewhat increased in size.
In the sections taken from the anterior part of the same embryo the posterior rudiment has both grown in size and also commenced to undergo histological changes by which it has become divided into a root, a ganglion, and a nerve.
The root (fig. D c, pr) consists of small round cells which lie close to the spinal cord, and ends dorsally in a rounded extremity.
The ganglion (g) consists of larger and more elongated cells, and forms an oval mass enclosed on the outside by the downward continuation of the root, having its inner side nearly in contact with the spinal cord.
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-embryos, of which I have a more complete series of sections than of the Torpedo-embryos.