SYMPATHETIC TRUNKS IN CHICK 345 



COXCLUSIOXS 



The observations recorded in the preceding pages demonstrate 

 clearly that the priniordia of the gangha of the s\Tnpathetic 

 trunks may arise in the absence of spinal gangUa and dorsal 

 nerve-roots; consequently, cells of medullary origin which advance 

 peripheral!}^ along the paths of the ventral roots of the spinal nerves 

 enter these priniordia. The spinal ganglia are not excluded as a 

 source from which cells may enter the primordia of the sympa- 

 thetic trunks under normal conditions; however, these primordia 

 ma}' arise from cells derived from the nem^al tube onl}-, at least 

 when cells which have their origin in the spinal gangha (or neural 

 crest) are excluded. 



As observed above, the primordia of the gangha of the sympa- 

 thetic trunks may be approxmiately of nonnal size in segments 

 in which the spinal gangha and dorsal nerve-roots are absent, 

 but the remnant of the neural tube is relatively large. On the 

 other hand, these primordia are small or entheh' absent in seg- 

 ments in which the remnant of the neural tube is small and repre- 

 sents only the most ventral portion of the central nervous system, 

 even though ventral nerve-roots are present. These facts sug- 

 gest that the cells which normall}- give rise to the gangha of the 

 sjTupathetic trunks are derived largely from those portions of the 

 walls of the neural tube which give rise to the lateral cell-columns. 

 Theoretical considerations also favor this interpretation; however, 

 we do not feel that the e\'idence at hand warrants a definite 

 conclusion on this point. 



