NERVOUS SYSTEM OF THE VERTEBRATA. 



451 



but it is nevertheless to be regarded as a very important morpho- 



logical structure. 



FIG. 266. SPINAL NERVES OF SCYI.LIUM 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 

 commissure 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. 



nc 



FIG. 267. SECTION THROUGH THE DORSAL PART OF THE TRUNK OF A 



TORPEDO EMBRYO. 



/>;-. posterior root of spinal nerve ; g. spinal ganglion ; n. nerve ; ar. anterior root 

 of spinal nerve; ch. notochord ; nc. neural canal; mp. muscle-plate. 



29 2 



