THE NERVOUS SYSTEM 257 



migrating from the cerebro-spinal ganglia to their definitive 

 positions. The first view was held by the earlier investigators 

 and was originally associated with the extinct idea that the spinal 

 ganglia were mesenchymal in origin; the view has been entirely 

 abandoned. The second view was partly established with the 

 discovery that the spinal ganglia are of ectodermal origin, and 

 that the ganglia of the main sympathetic trunk arise from the 

 spinal ganglia; but there is some difference of opinion yet in 

 regard to the peripheral ganglia of the symphathetic system, 

 and especially the plexuses of Meissner and Auerbach in the walls 

 of the intestine. However, the preponderance of evidence and 

 logic favors the view of the ectodermal origin of the entire sym- 

 pathetic nervous system. 



The first clear evidences of the sympathetic nervous system 

 of the chick are found at about the end of the third or the begin- 

 ning of the fourth day; at each side of the dorsal surface of the 

 aorta there is found in cross-section a small group of cells massed 

 more densely than the mesenchyme and staining more deeply. 

 Study of a series of sections shows these to be a pair of longi- 

 tudinal cords of cells beginning in the region of the vagus, where 

 they lie above the carotids, and extending back to the beginning 

 of the tail; the cords are strongest in the region of the thorax, 

 and slightly larger opposite each spinal ganglion. Cells similar 

 to those composing the cords are found along the course of the 

 nerves up to the spinal ganglia, and careful study of earlier stages 

 indicates that the cells composing the cords have migrated from 

 the spinal ganglia. The two cords constitute the primary sym- 

 pathetic trunks. 



Fig. 152 is a reconstruction of the anterior spinal and sym- 

 pathetic ganglia of a chick embryo of four days. The primary 

 sympathetic trunk is represented by a cord of cells enlarged 

 opposite each ganglion and united to the spinal nerve by a cellu- 

 lar process, the primordium of the ramus communicans. In the 

 region of the head the segmental enlargements are lacking. 



No other part of the sympathetic nervous system is formed 

 at this time with the exception of a group of cells situated in the 

 dorsal mesentery above the yolk-stalk; these are destined to 

 form the ganglion and intestinal nerves of Remak. They have 

 not been traced back to the spinal ganglia, but it is probable 

 that such is their origin. 



