SYMPATHETIC SYSTEM IN BIRDS 



287 



appearance occurs first in the thoracic region, while the last 

 remnants may be observed in the anterior cervical region. 



(c.) Secondary sy?npathetic trunks. — The anlagen of the secon dary 

 sympathetic trunks arise about the beginning of the sixth day 

 (120-hour stage), as ganglionic enlargements on the median sides 

 of the spinal nerves at the point of origin of the communicating 



spn. 



ic r m. sp.ii. 



psy LC.c.r. 



A 



C 





<s •* ssy sp.n. 



Wp-W 







\ ■« &. 



D 



i 



«y 



FSjVl. 



Fig. 2. Transverse sections showing successive stages in the development of 

 the primary and the secondary sympathetic trunks in the chick. A., primary 

 sympathetic trunk (96 hours incubation), X 200. B., Primary sympathetic trunk 

 (105 hours incubation), X 200. C, Primary and secondary sympathetic trunks 

 (120 hours incubation), X 200. D., Secondary sympathetic trunk (144 hours in- 

 cubation), X 100. ao., aorta; c.r., communicating ramus; i.c.c.r., cells migrating 

 from spinal nerve to primary sympathetic trunk; i.c.n., indifferent cell undergoing 

 mitosis; p.sy., primary sympathetic trunk; sp.n., spinal nerve; s.sy., secondary 

 sympathetic trunk. 



