286 ALBERT KUNTZ 



Figure 1 has been introduced to show the relative positions of 

 the primary and the secondary sympathetic trunks in an embryo 

 of the chick in the 130- hour stage. 



(b.) Primary sympathetic trunks.- — The primary sympathetic 

 trunks arise about the beginning of the fourth day of incubation, 

 as cell-aggregates lying along the sides of the aorta and along the 

 dorsal surfaces of the carotid arteries. At the close of the fourth 

 day (96- hour stage) , these cell-aggregates have assumed the appear- 

 ance of loosely aggregated cell-columns (fig. 2, A, p. sy.). Well 

 marked ganglionic enlargements do not occur, but the cell-col- 

 umns are not of uniform diameter. In the posterior region, the 

 anlagen of the primary sympathetic trunks arise a little later than 

 in the anterior region, and remain less sharply limited. They are, 

 at this stage, not directly connected with the spinal nerves. In 

 the thoracic region where the spinal nerves are best developed, 

 they extend peripherally a little beyond the level of the aorta. 

 At a point a little above the level of the aorta, cells deviate from 

 the course of the spinal nerves and wander through the mesen- 

 chyme, either singly or in small groups, toward the sides of the 

 aorta (fig. 2, A and B, i. c. c. r.) where they become aggregated to 

 give rise to the anlagen of the primary sympathetic trunks. 



During the course of the fifth day of incubation, the primary 

 sympathetic trunks become more conspicuous. They move dor- 

 sally and recede a short distance from the walls of the aorta until 

 at the close of the fifth day (120-hour stage) they appear as con- 

 spicuous cell-columns lying along the dorso-lateral aspects of the 

 aorta a short distance from its surface (fig. 2, C, p. sy.). The pri- 

 mary sympathetic trunks are now sharply defined in the anterior 

 region and are connected with the spinal nerves by distinct cel- 

 lular tracts. In the posterior region, the cell-aggregates are still 

 loosely scattered along the sides of the aorta and the cellular 

 tracts connecting them with the spinal nerves are less distinct. 

 The primary sympathetic trunks have now reached their maxi- 

 mum development. During the course of the sixth day, they 

 decrease materially in size until at the close of the sixth day (144- 

 hour-stage) they have almost disappeared. Their complete (lis- 



