152 WILLIAM F. ALLEN 



root appears to furnish the majority of fibers for the dorsal 

 motor ramus, and the cephalic root the bulk of fibers for the 

 ventral motor ramus, suggesting, possibly, that the dorsal and 

 ventral motor rami of more primitive vertebrates may have 

 arisen separately from the spinal cord. There is, however, 

 nothing in the development of the spinal nerves in Polistotrema 

 or in the adult condition of Petromyzon or Amphioxus to sup- 

 port this supposition. 



A comparison of the reconstruction of seven of the spinal 

 nerves in the region of the caudal heart of an 85 mm. Polisto- 

 trema embryo (fig. 3) with figure 1 discloses very few differ- 

 ences. Those that occur are apparently embryonic. The fact 

 that the motor and sensory roots run for relatively shorter dis- 

 tances in the membranous neural canal can probably be attributed 

 to a more rapid growth of the skeletal axis and myotomes over 

 the neural axis. Also it will be seen from figure 3 that fewer 

 of the motor and sensory rami have joined in the region of the 

 M. cordis caudalis. This union has taken place only in the 

 first nerve in reconstruction 3, while in reconstruction 1 it oc- 

 curred in the first three nerves that cross the muscle; the motor 

 rami of the remaining nerves in both reconstructions cross the 

 sensory rami without any exchange of fibers. As in reconstruction 

 1, ganglion cells are present along the course of some of the nerves. 

 A cluster of cells is shown encircling four out of seven of the 

 dorsal sensory rami at the base of the dorsal fin. 



In the reconstruction of three spinal nerves (fig. 4) taken from 

 the region of the caudal end of the M. cordis caudalis of an 

 adult Polistotrema it will be seen that there is almost no dif- 

 ference in the distribution and arrangement of the sensory and 

 motor fibers when compared with similar nerves taken from 

 reconstructions 1 and 3. A branch (R.M.C.C.) from the next 

 to the last motor ramus that crosses the M. cordis caudalis 

 enters this muscle and apparently innervates it, but no branch 

 was found going to the inner surface of the muscle as was de- 

 scribed for reconstructions 1 and 3. Whether this branch was 

 absent in this specimen or undifferentiated was not determined. 

 Isolated cells and clusters of gangUon cell are visible along the 



