SYMPATHETIC NERVOUS SYSTEM IN MAN 189 



distinct vagus branch accompanied by migrant cells may be 

 traced into proximity with the bulbar region of the heart. This 

 nerve gradually becomes larger and cells become aggregated near 

 its distal end. In an embryo 14 mm. in length (no. 511) it shows 

 a considerable accumulation of these migrant cells near its distal' 

 extremity, as illustrated in the accompanying figure (fig. 4) which 

 is taken from the microphotograph of a sagittal section shown in 

 figure 15. This mass of cells constitutes a portion of the pri- 

 mordium of the cardiac plexus. Over the dorsal aspect of the 

 heart the plexiform network around the oesophagus comes into 

 very close proximit}^ with the atrial walls. In transverse sec- 

 tions of an embryo 10.1 mm. in length (no. 623) the oesophageal 

 plexus is very conspicuous (figs. 5, 18, and 19). The branches 

 of the vagi with their accompanying cell-aggregates form a plexi- 

 form network including many cell-masses which is somewhat 

 richer over the dorsal and lateral aspects of the oesophagus 

 than between the latter and the trachea. Below the bifurcation 

 of the trachea, vagus fibers accompanied by cells of cerebrospinal 

 origin may be traced into the roots of the lungs where masses 

 of cells of the same type occur in proximity with the bronchi and 

 the pulmonary vessels (figs. 6, pp, and 20, pp). These plexiform 

 networks which constitute the primordia of the pulmonary plex- 

 uses are continuous with the oesophageal plexus as well as with 

 that portion of the cardiac plexus which is associated with the 

 atrial walls. 



In the writer's preparations of embryos of the pig 6 to 12 mm. 

 in length the oesophageal plexus is less conspicuous than in human 

 embryos of corresponding stages; however, vagus branches ac- 

 companied by migrant cells of nervous origin are present in con- 

 siderable abundance around the oesophagus. By reason of the 

 proximity of many of these elements with the oesophageal epi- 

 thelium, they were erroneously interpreted as representing mainly 

 the primordia of the enteric plexuses. The same error in inter- 

 pretation, though it is less apparent, occurs also in the writer's 

 studies of the development of the sympathetic nervous system 

 in the other classes of vertebrates. The primordia of the oeso- 

 phageal plexus are present before either nerve cells or fibers 



