SYMPATHETIC SYSTEM IN BIRDS 305 



the prevertebral plexuses show their maximum development in the 

 region of the suprarenals. In the early stages in the chick, these 

 plexuses show their maximum development in the sacral region. 

 This character in birds is obviously correlated with the enormous 

 development of the ganglion of Remak which has no counter- 

 part in mammals. Minor differences also occur in the develop- 

 ment of the vagal sympathetic plexuses. These morphogenetic 

 differences, doubtless, indicate that the sympathetic system has 

 departed more widely from the ancestral type in birds than in 

 mammals. 



A study of the development of the sympathetic system in birds, 

 as well as in mammals, warrants the conclusion that the nervous 

 system is a unit of which the sympathetic system is a part homol- 

 ogous with the other functional divisions. It may be looked 

 upon as one of the later accessions to the vertebrate nervous 

 system which has arisen in response to the conditions of the 

 vegetative life. The morphogenetic differences which have been 

 pointed out in the development of the sympathetic system in 

 birds and in mammals obviously indicate specializations in cer- 

 tain directions, which have arisen in response to peculiar vege- 

 tative functions. 



SUMMARY 



1. The primary sympathetic trunks in the chick arise about 

 the beginning of the fourth day of incubation, as a pair of cell- 

 columns lying along the sides of the aorta and along the dorsal 

 surfaces of the carotid arteries. The anlagen of the secondary 

 sympathetic trunks arise about the beginning of the sixth day, 

 as ganglionic enlargements on the median sides of the spinal 

 nerves. These ganglionic enlargements are at first independent 

 of each other, but become united later by longitudinal commis- 

 sures. The primary sympathetic trunks reach their maximum 

 development during the course of the sixth day, after which they 

 decrease in size until they disappear. The observations just 

 summarized agree essential^ with the results of His, Jr. 



