NERVOUS SYSTEM OF THK YKRTEHRATA. 



371 



but it is nevertheless to be regarded as a very important morphological 

 structure. 



B 



FIG. 266. SPINAL NERVES OF SCYLLIUM IN LONGITUDINAL SECTION TO SHEW 

 THE COMMISSURE CONNECTING THEM. 



A. Section through a series of nerves. 



B. Highly magnified view of the dorsal part of a single nerve, and of the com- 

 missure connected with it. 



com. commissure; sp.g. ganglion of posterior root ; ar. anterior root. 



It is probable, though the point has not yet been definitely made 

 out, that this commissure is derived from the neural crest, which 

 appears therefore to separate into two cords, one connected with each 

 set of dorsal roots. 



Returning to the original attachment of the nerve-rudiments to 

 the medullary wall, it has been already stated that this attachment 

 is not permanent. It 

 becomes, in fact, at about 

 the time of the appear- 

 ance of the above com- 

 missure, either extremely 

 delicate or absolutely in- 

 terrupted. 



The nerve - rudiment 

 now becomes divided into 

 three parts (figs. 267 and 

 268), (1) a proximal 

 rounded portion, to which 

 is attached the longitu- 

 dinal commissure (pr] ; 



(2) an enlarged portion, 

 forming the rudiment of 

 a ganglion (g and spg); 



(3) a distal portion, form- 

 ing the commencement 

 of the nerve (/?). The 

 proximal portion may 

 very soon be observed to 

 be united with the side 

 of the spinal cord at a 



</ 



FIG. 267. SECTION THROUGH THE DORSAL PART 

 OF THE TRUNK OF A TORPEDO EMBRYO. 



pr. posterior root of spinal nerve; g. spinal gan- 

 glion; 77. nerve; ar. anterior root of spinal nerve; 

 ch. notochord ; no. neural canal; mp. muscle-plate. 



242 



