SYMPATHETIC SYSTEM IN BIRDS 



299 



fibrous connections are established between the former and the 

 latter. There can be little doubt, therefore, that most of the cells 

 taking part in the development of the myenteric and the submuc- 

 ous plexuses in the walls of the small intestine migrate posteriorly 

 from the anlagen of these plexuses in the anterior region of the 

 digestive tube. 



(c.) Pulmonary plexuses. — In transverse sections through the 

 region of the lungs of embryos in the 144-hour stage, fibers may 

 be traced laterally from the branches of the vagi lying between 

 the oesophagus and the bronchi. Cells wander out along these 



, @& 



P&fll§#l 



vag. b 



Fig. 8. Transverse section through the region of the lungs of am embryo of the 

 chick (seventh day of incubation), X 80. ao., aorta; oe., oesophagus; p.p., anlagen 

 of pulmonary plexuses; vag. b., branches of the vagi; vag.n., vagus trunks. 



fibers and become aggregated to give rise to the anlagen of the pul- 

 monary plexuses, (fig. 8, p.p.). 



(d.) Cardiac plexus. — In transverse sections through the region 

 of the head in embryos in the 120-hour stage, cells may be traced 

 ventrally from the vagi into the septum of the atria where they 

 become aggregated into small groups which constitute the anlagen 

 of the cardiac plexus. In later stages, these cell-groups become 

 more conspicuous until at the close of the sixth day they appear as 

 distinct cell-aggregates in the atrial septum (fig. 9, car. p.). 



