SYMPATHETIC SYSTEM IN MAMMALS 227 



migrant cells move quite directly outward from the ventral zone, 

 while others tend ventro-laterally from the region in which later 

 the lateral horn of the gray matter arises. 



These migrant medullary cells, with similar elements which 

 wander out from the spinal ganglia, migrate peripherally along 

 the fibers of the spinal nerves. In transverse sections of embryos 

 7 or 8 mm. in length, the spinal ganglia are not sharply limited 

 distally. Cells become separated from their distal ends and mi- 

 grate peripherally along the fibers of the sensory roots. There 

 is no recognizable difference between the cells which wander 

 down from the spinal ganglia and those which migrate out from 

 the neural tube. It is impossible, therefore, to distinguish the 

 cells which migrate from the neural tube along the ventral nerve- 

 roots from those which become separated from the spinal ganglia, 

 after they have advanced beyond the point of union of the sen- 

 sory and motor roots. These "accompanying" cells are present 

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

 Fibers are not present as yet in the communicating rami, but 

 at a point a little above the level of the aorta, cells, either singly 

 or in small groups, deviate from the course of the spinal nerves 

 nearly at right angles and migrate through the mesenchyme to- 

 ward the dorso-lateral surfaces of the aorta, along the paths later 

 occupied by the fibers of the communicating rami (fig. 6, i.c.c.r.). 

 These findings do not differ essentially from those of Froriep. 

 They differ from those of the older investigators primarily in 

 the fact that cells migrate peripherally not only from the spinal 

 ganglia but also from the neural tube along the fibers of the ventral 

 nerve-roots. 



It is important to note at this point that the cells accompany- 

 ing the fibers of the spinal nerves actually migrate peripherally. 

 Fig. 7 indicates schematically the course and the direction of the 

 cells which migrate along the spinal nerves and the communicat- 

 ing rami into the sympathetic anlagen. Fig. 8 is designed to 

 show approximately the relative number of " accompanying " 

 cells present in the spinal nerves in successive stages during the 

 period of migration, and also the relative number remaining in the 

 nerve-trunks after migration has ceased. The figures in the hori- 



