450 



SPINAL NERVES. 



W 



-ao 



FIG. 264 A. TRANSVERSE SEC- 

 TION THROUGH A PRISTIURUS EM- 

 BRYO SHEWING THE PROLIFERATION 

 OF CELLS TO FORM THE NEURAL 

 CREST. 



pr. neural crest ; nc. neural canal ; 

 ch. notochord ; ao. aorta. 



FIG. 2646. TRANSVERSE SEC- 

 TION THROUGH THE TRUNK OF 

 AN EMBRYO SLIGHTLY OLDER 

 THAN FIG. 28 E. 



nc. neural canal ; /;-. posterior 

 root of spinal nerve ; x. subnoto- 

 chordal rod ; ao. aorta ; sc. so- 

 matic mesoblast ; sp. splanchnic 

 mesoblast ; mp. muscle-plate ; 

 mp'. portion of muscle-plate con- 

 verted into muscle ; Vv. portion 

 of the vertebral plate which will 

 give rise to the vertebral bodies ; 

 al. alimentary tract. 



before describing the further fate 

 of the nerve-rudiments it is ne- 

 cessary to say a few words as to 

 the neural crest. At the period 

 when the nerves have begun to 

 shift their attachment to the 

 spinal cord, there makes its ap- 

 pearance, in Elasmobranchii, a 

 longitudinal commissure con- 

 necting the dorsal ends of all 

 the spinal nerves (figs. 265, 266 

 coni), as well as those of the 

 vagus and glosso-pharyngeal 

 nerves. This commissure has 

 as yet only been found in a com- 

 plete form in Elasmobranchii ; 



FIG. 265. VERTICAL LONGITUDINAL 

 SECTION THROUGH PART OF THETRUNK 

 OF A YOUNG SCYLLIUM EMBRYO. 



com. commissure uniting the dorsal 

 ends of the posterior nerve-roots; pr. 

 ganglia of posterior roots ; ai: anterior 

 roots; st. segmental tubes; sd. segmental 

 duct ; g.e. epithelium lining the body 

 cavity in the region of the future germinal 

 ridge. 



