176 WILLIAM F. ALLEN 



Amphioxus. (f) From ordinary haematoxylin preparations the 

 peripheral nerve cells have the appearance of being bipolar, 

 resembling in every particular the true spinal ganglion cells. 

 (g) The distribution of these peripheral ganglion cells in the 

 adult and late embryo, and the fact that the most dorsal cells 

 of a spinal ganglion are more diffuse than the central or ventral 

 cells suggest that the peripheral cells had migrated from the 

 original neural crest. 



The writer would regard the arrangement of the scattered 

 peripheral nerve cells in the caudal sensory nerves, the isolated 

 and clustered peripheral cells in some of the more cephalic 

 nerves of Polistotrema as representing a tendency to repeat the 

 scattered disposition of the spinal ganglion cells found in the 

 simpler vertebrates of which Amphioxus is a type. This is 

 supported by the fact that in the simplest or most generalized 

 part of the nervous system of Polistotrema, namely, in the ex- 

 treme caudal portion, the peripheral nerve cells are abundant 

 throughout the course of the sensory rami, while they are col- 

 lected in small masses further cephalad, and still further cephalad 

 in the abdominal region, where the central nervous system be- 

 comes most specialized, only an occasional isolated nerve cell is 

 to be found along the course of some of the sensory nerves. 

 This scattered arrangement of the nerve cells throughout the 

 spinal nerves of Amphioxus and some of the nei'ves of Polisto- 

 trema is doubtless very old phylogenetically, dating back, pos- 

 sibly, to a still more diffuse arrangement of nerve cells found in 

 some of the invertebrates. 



The opposing view, which receives less support, would re- 

 gard these peripheral nerve cells in the spinal nerves as the anlage 

 of the sympathetic system. If this conjecture is true, the periph- 

 eral migration of the cells should be more pronounced in the trunk 

 region than in the tail region of Polistotrema, and would take 

 place in the ventral rami rather than in the dorsal; while, as a 

 matter of fact, this migration of nerve cells takes place mainly 

 in the dorsal rami in Polistotrema. It is very doubtful if a re- 

 layed visceral or sympathetic system occurs in the trunk region 

 of Amphioxus and Polistotrema, although continuous visceral 



