SYMPATHETIC SYSTEM IN BIRDS 291 



these trunks in the anterior region of the body are withdrawn 

 into the anlagen of the secondary or permanent sympathetic 

 trunks along the cellular tracts connecting the former with the 

 latter. The last remnants of the primary sympathetic trunks in 

 the anterior cervical region, as His, Jr., has suggested, probably 

 atrophy. 



The period of incubation being comparatively shorter in birds 

 than in mammals, cell migration takes place much more rapidly. 

 It is at its height in the chick during the fourth and the fifth day 

 of incubation. During this time breaches occur frequently in 

 the external limiting membrane of the neural tube just opposite 



c 





Fig. 4. Neuroblasts drawn with the aid of the camera lucida, X 825. a., in ven- 

 tral nerve-root inside external limiting membrane (105 hours incubation); b., in 

 ventral nerve-root outside external limiting membrane (105 hours incubation); 

 c, in spinal nerve (105 hours incubation); t/., in communicating ramus (105 hours 

 incubation); e., in ventral nerve-root (96 hours incubation): /., in spinal nerve 

 (96 hours incubation). 



the motor niduli, and medullary cells may be traced without diffi- 

 culty from the motor niduli into the proximal part of the ventral 

 nerve-roots (fig. 3, c. m. v. r.). Numerous accompanying cells are 

 present in the spinal nerve-trunks as far as the latter may be traced. 

 At the close of the sixth day, the number of cells present in the 

 spinal nerves has materially decreased. While cells are still 

 moving peripherally along the spinal nerves, it is probable that 

 migration from the neural tube and the spinal ganglia has prac- 

 tically ceased. 



The great majority of the cells migrating peripherally along 

 the spinal nerves are characterized b} r very little cytoplasm, and 



