370 



XKUVJW. 



-pr 



-UV 



FIG. 204 A. TKANSVEBSE SEC- 

 TION THROUGH A PKISTIUHUS EM- 

 BRYO SHEWING THE PROLIFEKATION 

 OF CELLS TO FORM THE NEU1JA1. 

 CREST. 



pr. neural crest ; nc. neural 

 canal; eft. notochord ; no. 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. 

 subuotochorclal rod ; ao. aorta ; 

 sc. somatic inesoblast ; sp. 

 splanchnic mesoblast ; mp. 

 muscle-plate ; mp'. portion of 

 muscle - plate converted into 

 muscle ; Yv. portion of the 

 vertebral plate which will give 

 rise to the vertebral bodies; /. 

 alimentarv trnH . 



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 ; st. segmental tubes ; stl. 

 segmental duct ; g.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 ; 



