SYMPATHETIC SYSTEM IN MAMMALS 215 



face of the aorta. These are the beginnings of the primary sympa- 

 thetic trunks. About the beginning of the sixth day the anlagen 

 of the secondary, or permanent, sympathetic trunks arise as a 

 series of cell-aggregates situated just median to the ventral roots 

 of the spinal nerves. The cells giving rise to the primary sym- 

 pathetic trunks migrate thither from the spinal ganglia, along the 

 spinal nerves and the communicating rami. The anlagen of the 

 secondary sympathetic trunks are separated from the spinal ganglia 

 only by the fibers of the ventral nerve-roots. Neuroblasts may 

 be found in the ventral nerve-roots, caught apparently in migra- 

 tion from the spinal to the sympathetic ganglia. After the appear- 

 ance of the secondary trunks, the primary sympathetic trunks 

 become resolved into the various ganglia and nerves of the pre- 

 vertebral and the peripheral sympathetic plexuses. This view 

 was adopted by Lillie ('08). 



Marshall ('93) found that the anlagen of the sympathetic trunks 

 in embryos of the frog and the chick arise "as a series of out- 

 growths from certain of the cranial and all of the spinal nerves." 

 These outgrowths develop into ganglionic enlargements which 

 become connected later by longitudinal commissures. These 

 findings agree essentially with Balfour's observations on the sela- 

 chians, but differ very materially from the findings of later obser- 

 vers for amphibians and birds. 



In his later researches on the urodeles, Hoffmann ('02) found 

 conditions of development differing widely from those in sela- 

 chians. In this type the anlagen of the sympathetic trunks arise 

 as scattered cells along the sides of the dorsal surface of the aorta, 

 some of which are connected with the communicating rami by 

 slender protoplasmic processes. 



In his work on the common toad, Jones ('05) pointed out a 

 notable difference in the development of the anterior and the pos- 

 terior regions of the sympathetic trunks. In the region anterior 

 to the second spinal nerve, they arise from cells scattered in the 

 mesoderm. This agrees essentially with the findings of Hoffmann. 

 In the region posterior to the second spinal nerve, ridges of cells 

 appear along the sides of the aorta. The cells at the tops of these 

 ridges become differentiated to form the sympathetic trunks. 

 These findings have not been substantiated by other observers. 



