312 OUTLINES OF CHORDATE DEVELOPMENT 



d 



n 



main branches (Fig. 126); first is given off ventrally a large 

 branch to the sympathetic system, the ramus communicans (see 

 below). These fibers are finally distributed to splanchno- 

 pleural derivatives, and are derived from both dorsal and 

 ventral roots. The trunk then separates into dorsal and ventral 

 branches, each containing fibers from both roots, which are 

 distributed chiefly, though not wholly, to somatopleural struc- 

 tures, the integument and stri- 

 ated muscles. Opposite the limb- 

 buds the spinal nerves accom- 

 pany extensions of the somites 

 into the limbs and provide their 

 innervation, supplying the mus- 

 cles and integument. 



Meanwhile the sympathetic 

 system has been forming. Dur- 

 ing the third day some of the 

 neuroblasts of the spinal ganglia 

 send out processes which extend 

 downward toward the aorta; 



FIG. 126. Diagram of the chief _,, , 



elements of the sympathetic nervous then along the paths thus marked 



system of the chick in transverse Qut gome f the neuro blastS of 

 section. After His, Jr. a, Dorsal 



aorta; ap, aortic plexus; d, dorsal the Spinal ganglia, and Others 

 (afferent) root of spinal nerve; g, c- ,-1 i , ., 1( , 

 spinal ganglion; i, intestine; m, from the S P mal C0rd ^elf, mi- 

 mesentery ;n, notochord;7Z, Remak's orate to a position ventral to the 



ganglion; s, splanchnic plexus; sg, , 



sympathetic elements in intestinal Spinal COl'd, ai'OUlld the dorsal 



wall; t, mesonephric tubules; v, aortfl TliPSP rniorfltnrv pplk 



ventral (efferent) root of spinal aC 



nerve; 7, primary sympathetic cord; tllUS establish a pair of loilgi- 

 //, secondary sympathetic cord. , . , 



tuduial cords, the primary sym- 

 pathetic cords (Fig. 126). Other migratory neuroblasts from 

 the spinal ganglia then form somewhat similarly, a pair of 

 secondary sympathetic cords just above the primary cords. 

 The secondary cords then become connected segmentally with 

 the spinal ganglia, by processes from the secondary cord cells 

 which extend back into the ganglia and through the dorsal spinal 

 root into the spinal cord. The primary sympathetic cords give 

 rise to the prevertebral plexuses, but otherwise disappear en- 



