SOMATIC MOTOR COLUMN OF AMBLYSTOMA 133 



the most rostral ohe or two, or possibly three myotomes without 

 interfering with the locomotor efficiency of the muscular system. 



In later stages of development the more rostral ventral roots 

 certainly receive collaterals also from descending processes of the 

 tract. This is correlated with the introduction of the cranial 

 sensory field through the descending trigeminal tract, the ven- 

 tral commissure and the extension rostrad of the motor column. 



It seems necessary, therefore, to regard the motor column at 

 the level of the most rostral myotomes to be both descending and 

 ascending in embryos of the early swimming stage. Just how 

 far caudad this ascending conduction may occur in the column 

 it is impossible to say at present. There are suggestions that 

 it advances caudad in development pari passu with the extension 

 caudad of the ventral commissure, but this is not demonstrable 

 with the material at hand. A more critical discussion of this 

 question must await the description of the sensory and commis- 

 sural systems. 



In the very early condition the axone of the tract bends out- 

 ward slightly at the origin of the root collateral (figs. 2, 4, 9, 11). 

 Later the outward deflection at this point affects numbers of 

 fibers and a large element of the tract bends outward in an abrupt 

 loop, from or near the tip of which the root collaterals arise 

 (figs. 22, 23, 24, 25, 27). This arrangement suggests Johnston's 

 figure of the dorsal roots of Amphioxus^ where fibers of the tract 

 deflect out in long loops into the dorsal roots. Johnston sug- 

 gests that this condition is produced by the outward migration 

 of cells from the cord along the roots. No such agency, however, 

 can account for the condition in Amblystoma embryos for during 

 the periods under consideration there is no perceptible migration 

 of cells from the cord into or along the roots. Furthermore, to 

 recur to the condition in Amphioxus, Johnston 'describes the 

 viscero-motor fibers as running some distance in the cord longi- 

 tudinally before they go out into the root. The idea that some 

 at least of these deflected fibers in Amphioxus may be motor 

 immediately suggests itself. Those long loops into the root may 



^ J. B. Johnston, The cranial and spinal ganglia and the viscero-motor roots 

 in Amphioxus; Biological Bulletin, vol. 9, no. 2, fig. 4. 



