312 OUTLINES OF CHORDATE DEVELOPMENT 



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 path&thus marked 



SSS.^Ai.SSSf SToS out some of the neuroblasts of 

 aorta; ap, aortic plexus; d, dorsal the spinal ganglia, and others 



(afferent) root of spinal nerve; g, / ,-t - ^ > . -./. 



spinal ganglion; t, intestine; m, from the Spinal COrd itself, mi- 

 mesentery ; n, notochordjfl, Remak's gra t e to a position ventral to the 



ganglion; s, splanchnic plexus; sg, , r 



sympathetic elements in intestinal Spinal COTO, around the dorsal 



wall; t, mesonephric tubules; v, o nr fo Thp^P Tniorfltnrv pplk 



ventral (efferent) root of spinal * Ca - ni g ra 



nerve; I, primary sympathetic cord; thus establish a pair of longi- 



//, secondary sympathetic cord. i -i 



tudinal 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- 



