370 



SPINAL NERVES. 



Fio. 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. 264 B. Transverse 



SECTION THROUGH THE TRUNK 

 OF AN EMBRYO SLIGHTLY OLDER 

 THAN FIG. 28 E. 



nc. neural canal; pr. pos- 

 terior root of spinal nerve ; x. 

 subnotochordal rod; ao. aorta; 

 sc. somatic mesoblast ; sp. 

 splanchnic mesoblast ; mp. 

 muscle-plate ; mp'. portion of 

 muscle - plate converted into 

 muscle ; Vv. portion of the 

 vertebral plate which will give 

 rise to the vertebral bodies; al. 

 alimentary tract. 



m sk i I 



Im fil Mi m > 



m ^' m m " 1/ 





Fig. 265. Vertical longitudinal 

 section through part of the trunk 

 of a young scyllium embryo. 



com. commissure uniting the dorsal 

 ends of the posterior nerve - roots ; 

 pr. ganglia of posterior roots ; ar. an- 

 terior roots ; at. segmental tubes ; sd. 

 segmental duct ; (i.e. epithelium lining 

 the body cavity in the region of the 

 future germinal ridge. 



nerves. While still remaining at- 

 tached to the dorsal summit of the 

 neural cord they grow to a consider- 

 able size (fig. 264 B, pr). 



The attachment to the dorsal 

 summit is not permanent, but before 

 describing the further fate of the 

 nerve-rudiments it is necessary 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 

 appearance, in Elasmobranchii, a 

 longitudinal commissure connecting 

 the dorsal ends of all the spinal 

 nerves (figs. 265, 266 com), as well 

 as those of the vagus and glosso- 

 pharyngeal nerves. This commis- 

 sure has as yet only been found in a 

 complete form in Elasmobranchii ; 



