SYMPATHETIC SYSTEM IN MAMMALS 217 



ion that all the sympathetic neurones in the sympathetic trunks 

 as well as in the prevertebral and the peripheral sympathetic 

 plexuses have their origin in the ventral half of the neural tube. 



Held ('09) and Marcus ('09) have recently taken exception to 

 Froriep's conclusions. Held has attempted to show, for the entire 

 vertebrate series, that the cells present in the motor nerve-roots 

 play no part in the development of the sympathetic system. He 

 still regards the sympathetic system as an offshoot from the spinal 

 ganglia. Marcus has attempted to show that the cell-groups 

 which Froriep observed in the ventral roots of the spinal nerves 

 do not wander out from the neural tube, but migrate thither from 

 the neural crest. In early stages of embryos of Torpedo, he has 

 observed cell-chains connecting the neural crest with the cell- 

 aggregates in the ventral nerve-roots. He concludes, therefore, 

 that the neural crest represents the sole source of all the cells 

 giving rise to sympathetic neurones. 



This brief review of the literature has shown that the advocates 

 of the theory of the ectodermal origin of the sympathetic nervous 

 system agree in tracing the origin of the cells giving rise to the 

 sympathetic anlagen to the cerebro-spinal system. There is a 

 wide difference of opinion, however, concerning the immediate 

 source and the histogenesis of these cells. 



Two views have been prevalent among the older investigators. 

 Onodi advanced the idea that the cells at the distal ends of the 

 spinal ganglia are forced to advance farther peripherally by the 

 pressure exerted by the newly formed elements back of them. In 

 this manner cell-groups become constricted off from the spinal 

 ganglia and give rise to the anlagen of the sympathetic trunks. 

 His, His, Jr., and some of the later writers have traced the origin 

 of the cells giving rise to the sympathetic anlagen to the spinal 

 ganglia, but have accounted for the transfer of these elements 

 from the spinal ganglia to their new location by active migra- 

 tion, either directly through the mesenchyme or along the paths 

 of the spinal nerves and the communicating rami. 



The difference between these two views may be accounted for 

 in part by fundamental differences in the morphogenesis of the 

 sympathetic nervous system in the various classes of vertebrates. 



