272 



Basic Structure of Vertebrates 



out nerve-fibers which constitute the dorsal sensory root of a spinal 

 nerve. The fibers of the other constituent root of a spinal nerve, the 

 ventral motor root, grow out from cells within the neural tube. Some 



Fig. 225. Diagrams illustrating method 

 of origin of the neural tube of verte- 

 brates. Transverse sections in the mid- 

 trunk region of embryos at successively 

 (A-D) later stages, (c) Neural crest; 

 (cc) canalis centralis of neural tube; 

 (ec) ectoderm; (en) endoderm; (mes) 

 mesoderm: (nc) notochord; (ng) neu- 

 ral groove; (np) neural plate; (nt) 

 neural tube; (v) blood-vessel (paired 

 dorsal aorta). (Courtesy, Neal and Rand, 

 "Chordate Anatomy," Philadelphia, 

 The Blakiston Company.) 



cells of the neural crests migrate into various visceral localities and 

 give rise to ganglions ("sympathetic": Fig. 157) and nerves of the 

 autonomic system. 



