176 DEVELOPMENT OF THE SPINAL NERVES 



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 possesstng 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 down- 

 ward 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 con- 

 siderable 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 Pristiitrus-embryos, of which I have a more com- 

 plete series of sections than of the Torpedo-embryos. 



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 under- 

 gone 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 



