132 G. E. COGHILL 



2. DISCUSSION OF THE RELATIONS IN GENERAL 



From the foregoing particular descriptions it is obvious that 

 neurones of the somatic motor column become well differentiated 

 and typically oriented in the spinal cord before the ventral roots 

 appear in the corresponding level. The earliest demonstrable 

 root fibers arise as collaterals from these neurones. The great 

 majority of clear cases indicate that the collaterals regularly arise 

 from descending processes excepting in the most rostral nerves. 

 In these nerves — the first, second and possibly the third pair — 

 the root collaterals may arise from ascending processes. 



This origin of the motor roots from descending processes of 

 tract neurones is in harmony with the general process of inte- 

 gration of the nervous system, which, in these early periods of 

 development, conducts, in the interest of locomotion, all stimuli 

 from the skin directly to the rostral end of the muscle system, 

 and thence caudad so as to produce a wave of contraction that 

 progresses cephalo-caudad. Since the neurones of the descending 

 tract are at the same time the neurones of the ventral roots 

 their polarization would require that root fibers arise from the 

 neurite, or descending process. In the region of the most rostral 

 myotomes, however, the neurites seem to ascend in the motor 

 column. While this may seem at first thought to be inconsistent 

 with the general plan of integration of the motor system it is 

 not so in reality, for, while it is necessary for locomotion that 

 the wave of contraction be stimulated in cephalo-caudal progres- 

 sion, the simultaneous contraction of several of the most rostral 

 myotomes of the same side would not interfere with the efficiency 

 of the movement. Such simultaneous contraction of myotomes 

 in the rostral region would tend to be produced by the introduc- 

 tion of ascending processes in the motor path to the myotomes, 

 for, in embryos of the early flexure stage, the sensory field lies 

 chiefly caudad, in the territory of the giant ganglion cells, and 

 the sensory path ascends to the region of the most rostral myo- 

 tomes before it can reach the motor system. This it accomplishes 

 at the level of the most rostral myotomes and from this center 

 the motor column can participate in the ascending conduction to 



