148 WILLIAM F. ALLEN 



the neural canal and a second dorsal branch arises in like man- 

 ner from the more cephalic stem of the caudal ventral roo to 

 unite with the first dorsal branch opposite the spinal cord in form- 

 ing the dorsal motor ramus. This ramus continues in a dorsal 

 direction in front of the corresponding dorsal sensory ramus and 

 entirety separate from it. In figure 5 a caudal branch will be 

 seen passing median to the dorsal sensory ramus, and terminat- 

 ing in that part of the myotome which is bounded by the dorsal 

 median septum, roof of the neural arch and dorsal sensory 

 ramus. 



Figures 1 to 4 show a very different arrangement for the sen- 

 sory and motor components of the spinal nerves for the tail 

 region. Most significant, is the fact, that the majority of the 

 motor and sensory rami remain separate throughout, resem- 

 bling Petromyzon in this respect. The above mentioned figures 

 include graphic reconstructions of some of the caudal spinal 

 nerves from a transverse series of a small adult Polistotrema, 

 from an 85 mm. Polistotrema embryo that had attained nearly 

 adult conditions, and from a full grown adult. 



In figure 1, where eleven of the last spinal nerves were recon- 

 structed, seven have separate motor and sensory rami. The 

 ventral motor rami of the last three nerves assume a gradual 

 caudal course in crossing the notochord so that they eventually 

 become located closer to the succeeding ventral sensory rami 

 than they do their own. The third from the last ventral motor 

 ramus follows the inner surface of a myotome about equidis- 

 tant between its sensory ramus and the following one. It ac- 

 companies a branch from its sensory ramus when on a level with 

 the lower surface of the caudal artery. The three most cephalic 

 of the last seveij ventral motor rami follow at first the inner sur- 

 face of the myotomes a short distance behind their respective 

 sensory rami until considerably below the level of the caudal 

 artery, then bend cephalad, cross the outer surface of their 

 sensory rami without any exchange of fibers and terminate in 

 the myotomes. A conspicuous cephalic branch (R.M.C.C.) is 

 given off from the last of these motor rami to pass along the 

 inner surface of the M. cordis caudalis for a distance of at least 



