SYMPATHETIC SYSTEM IN BIRDS 



285 



day of incubation, as a pair of cell-columns lying along the sides 

 of the dorsal surface of the aorta. About the beginning of the 

 sixth day, the anlagen of the secondary sympathetic trunks arise 

 as cell-aggregates situated just median to the ventral roots of 

 the spinal nerves. These cell-aggregates are at first independent of 

 each other, but become united later by longitudinal commissures. 

 Between the fourth and the eighth day of incubation, the primary 

 sympathetic trunks disappear, except in the most anterior region, 

 becoming resolved into the ganglia and nerves constituting the 

 prevertebral and the peripheral sympathetic plexuses. According 



Fig. 1. Diagramatic transverse section through the thoracic region of an embryo 

 of the chick (130 hours incubation), ao., aorta; nc., notochord; p.sy., primary 

 sympathetic trunks; sp.g., spinal ganghon; sp.n., spinal nerve; s.sy., secondary 

 sympathetic trunks. 



to His, Jr., the cells giving rise to both the primary and the sec- 

 ondary sympathetic trunks are derived exclusively from the 

 spinal ganglia. 



My observations on the development of the sympathetic trunks 

 in the chick do not differ essentially from those of His, Jr., except 

 in one particular. I find that the cells giving rise to the primary 

 and the secondary sympathetic trunks in the chick, like the cells 

 giving rise to the sympathetic trunks in mammals, are not derived 

 exclusively from the spinal ganglia, as His, Jr., believes them to be, 

 but that they have their origin, wholly or in part, in the neural 

 tube. 



