VI.] THE SPINAL NERVES. 131 



Fib. 44. 



pr 



mp 



Transverse section through the trunk op a young embryo 

 OP A Dog-Fish. 



nc. neural canal ; pr. posterior root of spinal nerve ; x. sub- 

 notoohordal rod ; oio. aorta ; so. somatic mesoblast ; sp. 

 splanchnio mesoblast ; Tivp. muscle-plate ; m/p'. portion of 

 muscle-plate converted into muscle ; Vv. portion of the 

 vertebral plate which will give rise to the vertebral bodies ; 

 al. alimentary tract. 



It is extremely difficult to decide whether the per- 

 manent attachment of the posterior nerve-roots to the 

 spinal cord is entirely a new formation, or merely due 

 to the shifting of the original point of attachment. 

 We are inclined to adopt the former view. 



The origin of the anterior roots of the spinal nerves 

 has not as yet been satisfactorily made out in Birds ; 

 but it appears probable that they grow from the ventral 

 comer of the spinal cord, considerably later than the 

 posterior roots, as a number of strands for each nerve, 



9—2 



