SPINAL NEKVES IN VERTEBRATES 163 



join in forming short mixed rami ventrales {R.V.). (b) A 

 conspicuous cluster of cells, the anlage of a vertebral or chain 

 ganglion (figs. 20 and 34, Sy.G.), is present on the direct course 

 of each sensory ramus ventralis, opposite the dorsal aorta. In 

 figures 20 and 34 sensory fibers (R.V.S.) presumably having 

 their cell bodies in a spinal ganglion will be seen leaving a sym- 

 pathetic gajiglion to join with the fibers of a corresponding motor 

 nerve in forming a mixed ramus ventralis. As was- stated for 

 an earlier reconstruction (fig. 17), it is difficult to see how in 

 Squalus many, if any, of the cells of neural tube origin found 

 along the outer surface of the motor rami ventrales would take 

 any part in the formation of sympathetic ganglia. In this re- 

 construction it is of interest to recall the case of Petromyzon, 

 where the motor and sensory rami ventrales are always separate, 

 in which Julian ('86) and ('87) found sympathetic cells about 

 the dorsal aorta, at the end of the rami communicantes, w^hich 

 are branches from the sensory rami ventrales. From this ar- 

 rangement in .the ammocoetes stage it is reasonable to suppose 

 that these sympathetic cells had their origin from the neural 

 crest and migrated to the aorta along the course of the above 

 mentioned nerves. It must be admitted that it is no easy tafek 

 to reconcile the origin of purely excitatory or effective sympa- 

 thetic cells from the neural crest, a supposedly purely receptive 

 area. Concerning the vertebral, prevertebral, and effective 

 peripheral ganglion cells it is conceivable that they may have 

 migrated out into the neural crest from an effective or motor 

 area of the neural tube, or they may have been indifferent cells, 

 or their function may have been modified. It would seem that 

 the principal requirement for a spinal visceral effective or motor 

 relay is that its primary cell in the gray matter of the spinal 

 cord should take origin from the effective or motor area of the 

 neural tube, (c) It is clear from figure 20 that each spinal 

 nerve at this stage has two ventral or motor roots (V.R.). It 

 was not determined whether this means the formation of addi- 

 tional fibers in another region or the splitting up of the original 

 root fibers into two portions, (d) A comparison with figure 17 

 shows that the distal portions of the dorsal and ventral rami 



