l8o DEVELOPMENT OF THE SPINAL NERVES 



In the best specimens a distinct bordering line can be seen, 

 and it is, as a rule, possible to state the characters by which 

 the cells of the nerve-rudiments and vertebral bodies differ. The 

 more important of these are the following: (i) The cells of 

 the nerve-rudiment are distinctly smaller than those of the 

 vertebral rudiment ; (2) the cells of the nerve-rudiment are 

 elongated, and have their long axis arranged parallel to the long 

 axis of the nerve-rudiment, while the cells surrounding them are 

 much more nearly circular. 



The cells of the nerve-rudiment measure about -^-^ x ^^ to 

 unnr x W<nr mcn > those of the vertebral rudiment y^ X TI| L^ inch. 

 The greater difficulty experienced in distinguishing the nerve- 

 rudiment from the connective-tissue in Pristiurus than in 

 Torpedo arises from the fact that the connective-tissue is much 

 looser and less condensed in the latter than in the former. 



The connective-tissue cells which have grown out from the 

 muscle-plates form a continuous arch over the dorsal surface of 

 the neural tube (vide PI. 22, fig. F) : and in some specimens 

 it is difficult to see whether the arch is formed by the rudiment 

 of the posterior root or by connective-tissue. It is, however, 

 quite easy with the best specimens to satisfy one's self that it is 

 from the connective-tissue, and not the nerve-rudiment, that the 

 dorsal investment of the neural canal is derived. 



As in the previous case, the upper ends of each pair of 

 posterior nerve-rudiments are quite separate from one another, 

 and appear in sections to be united by a very narrow root 

 to the walls of the neural canal at the position indicated in 

 fig. F 1 . 



The cells forming the nerve-rudiments have undergone slight 

 modifications ; they are for the most part more distinctly elon- 

 gated than in the earlier stage, and appear slightly smaller in 

 comparison with the cells of the neural canal. 



They possess as yet no distinctive characters of nerve- 

 cells. They stain more deeply with osmic acid than the cells 

 around them, but with haematoxylin there is but a very slight 

 difference in intensity between their colouring and that of the 

 neighbouring connective-tissue cells. 



The anterior roots have grown considerably in length, but 



1 The artist has not been very successful in rendering this figure. 



