SPINAL NERVES IN VERTEBRATES 175 



repeat in part the condition found in more primitive forms, as 

 for example Amphioxus. 



In a transverse section (fig. 10) taken several segments behind 

 the above reconstruction a considerable number of nerve cells 

 (N.C), enough to constitute a ganglion, are located on the sen- 

 sory ramus dorsalis, immediately dorsal and lateral to the myo- 

 tomes. In these haematoxylin preparations the peripheral 

 nerve cells did not differ from the ordinary spinal ganglion cells. 



A reconstruction of seven spinal nerves in the caudal heart 

 region of an 85 mm. Polistotrema embryo (fig. 3) shows a cluster 

 of nerve cells (N.C.) on the peripheral course of the first, second, 

 third and fifth dorsal sensory rami, adjacent to the median dor- 

 sal bar, and the second dorsal sensory ramus has in addition a 

 nerve cell in the dorsal fin region. The ventral rami of these 

 nerves, in contrast, possess only one peripheral nerve cell, which 

 is situated in the last ventral sensory ramus opposite the noto- 

 chord. 



The following generalizations can be made for certain periph- 

 eral ganglion cells found along the course of the spinal nerves 

 of Polistotrema. (a) Nerve cells appear isolated or in clus- 

 ters along the peripheral course of many spinal nerves (b) 

 It is uncommon to find a peripheral nerve cell in a ventral sen- 

 sory ramus or a mixed ramus ventralis. Out of twenty-four 

 typical rami ventrales reconstructed, if the last two nerves are 

 eliminated, but three peripheral nerve cells were found, and no 

 nerve contained more than one cell. One of these cells was 

 situated opposite the notochord and the other two were about 

 on a level with the lower border of the myotomes, (c) None of 

 the cephalic sensory rami dorsales reconstructed possessed 

 any peripheral nerve cells; while in the caudal region they are 

 common, nevertheless over one-half have no cells, (d) In the 

 region of the caudal heart the most common place for the periph- 

 eral nerve cells in the dorsal sensory rami is opposite the 

 median dorsal bar; they frequently occur above the dorsal 

 border of the myotomes, (e) Scattered nerve cells are found 

 everywhere in the plexus of nerves ending in the last two spinal 

 ganglia. Their distribution suggests the arrangement found in 



