166 WILLIAM F. ALLEN 



plane. In like manner a motor ramus ventralis (R.V.M.) 

 approaches its corresponding sensory ramus (R.V.S.) from in 

 front, a little below the level of its spinal ganglion, and the two 

 pursue a parallel and general caudo-ventral course along the 

 inner surface of the intermuscular septum, to a level with the 

 dorsal aorta, where they unite in forming a mixed ramus ven- 

 tralis {R.V.). Figure 27 discloses the relationship of the motor 

 and sensory rami ventrales in transverse section on a level with 

 the dorsal aorta. Apparently each spinal nerve has two ventral 

 or motor roots within the neural canal, which may take origin 

 from one or two rootlets. Upon leaving the neural canal these 

 roots enter the two arms of a Y-shaped foramen and emerge 

 outside as a single root, which soon separates into a dorsal and 

 a ventral motor ramus. 



Some of the important points of the previous section may be 

 summarized as follows: (a) In a description of a reconstruction 

 of several of the lower abdominal spinal nerves of a 7.5 mm. 

 Squalus it was shown that while the neural crest was engaged in 

 the formation of spinal ganglia, motor fibers and embryonic 

 neurilemma cells had left the ventro-lateral surface of the neural 

 tube and had grown laterally to the inner surface of the myo- 

 tomes, there to turn ventrad a short distance as motor rami 

 ventrales. From one of these nerves a few motor ramus dor- 

 salis fibers were given off to the inner surface of the myo- 

 tomes. Meanwhile the neural crest had proHferated and seg- 

 mentally arranged portions of it had migrated ventrally be- 

 tween the motor roots, there to bend toward the more cephalic 

 roots and motor rami ventrales as if by some attraction. These 

 ventral proliferations of the neural crest represent the anlage 

 ganglia and probably the beginnings of the sensory rami ven- 

 trales. (b) In the reconstruction of a later stage, taken from 

 more cephalic nerves of the same series, it was noted that the 

 sensory rami ventrales had made more rapid growth than the 

 motor rami ventrales and had extended ventrally to the level 

 of the dorsal aorta, where at their distal ends there was an 

 assemblage of cells, representing, doubtless, the anlage of the 

 vertebral or chain sympathetic ganglia. For the greater part 



