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 
largely, but not 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 sym- 
pathetic trunk arise from the spinal ganglia; but there is some 
difference of opinion yet in regard to the peripheral ganglia 
of the sympathetic 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 sympathetic 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. 
