184 DEVELOPMENT OF THE SPINAL NERVES 



whole spinal cord, except at the points of junction of the cord 

 with the nerve-rudiments. Externally to this investment, and 

 separated from it for the most part by a considerable interval, a 

 mesoblastic sheath (PL 23, fig. H I, i) for the spinal cord is 

 beginning to be formed. 



The attachment of the nerve-rudiments to the spinal cord, on 

 account of its smallness, it still very difficult to observe. In 

 many specimens where the nerve is visible a small prominence 

 may be seen rising up from the spinal cord at a point cor- 

 responding to x (PI. 23, fig. H l). It is, however, rare to see 

 this prominence and the nerve continuous with each other : 

 as a rule they are separated by a slight space, and frequently 

 one of the cells of the mesoblastic investment of the spinal cord 

 is interposed between the two. In some especially favourable 

 specimens, similar to the one figured, there can be seen a dis- 

 tinct cellular prominence (fig. H I, x) from the spinal cord, 

 which becomes continuous with a small prominence on the 

 lateral border of the nerve-rudiment near its free extremity. 

 The absence of a junction between the two in a majority of 

 sections is only what might be expected, considering how minute 

 the junction is. 



Owing to the presence of the commissure connecting the 

 posterior roots, some part of a nerve is present in every section. 



The proximal extremity of the nerve-rudiment itself is com- 

 posed of cells, which, by their smaller size and a more circular 

 form, are easily distinguished from cells forming the ganglionic 

 portion of the nerve. 



The ganglionic portion of the nerve, by its externally swollen 

 configuration, is at once recognizable in all the sections in 

 which the nerve is complete. The delicate investment before 

 mentioned is continuous around it. The cells forming it are 

 larger and more elongated than the cells forming the upper por- 

 tion of the nerve-rudiment : each of them possesses a large and 

 distinct nucleus. 



The remainder of the nerve rudiment forms the commence- 

 ment of the true nerve. It can in this stage be traced only for a 

 very small distance, and gradually fades away, in such a manner 

 that its absolute termination is very difficult to observe. 



The connective-tissue cells which surround the nerve-rudi- 



