184 Papers from the Marine Biological Laboratory at Tortugas. 



The 42-day larva shows the best development of these electric-motor- 

 nerve cells until an older fish can be described. Here we find a heavy, 

 rounded, cytoplasmic body of about 40 microns in diameter (fig. 14, plate 4, 

 A, B, and C). Its outline is usually pear-shaped, with the axis-cylinder 

 process given off at the pointed end. Dendrites are not visible in the 

 specimen at hand. While most of these cells are of the same size, a few 

 are noticeably smaller. 



The nucleus is large, round, and placed nearly always in a very eccentric 

 position. This position is most frequently on the side away from the 

 neurite, but sometimes it may be very close to the neurite. Its diameter 

 is about 16 to 19 microns in the largest cells of this age, and its outline is 

 hard and round. The nucleolus consists of a single, or rarely double, plas- 

 mosome, which is a very little less than 5 microns in diameter, but which, 

 when double or multiple, is of proportionally smaller size. As I have shown 

 in the nucleus of the electric cells of several torpedoes, when a plasmosome 

 is multiple its several parts, collectively, are larger in mass than is a single 

 normal or usual plasmosome. 



Of course, the nucleus was carefully examined as to any possible ori- 

 entation of its nuclei, particularly the plasmosome, with reference to gravity 

 or to the electric current or to the axis of the cell. Nothing of this sort 

 could be found, although this does not preclude such a condition in the 

 grown fish. It is known that in Torpedo, where such an orientation does 

 exist, this sam.e orientation is not found in the embryonic or larval stages. 



From these cells thin, delicate axis-cylinder processes were traced down, 

 as has been described, and into the ventral roots of the spinal nerves in a 

 sufficient number of cases to assure the observer that they all ran in this 

 direction. The process was thinnest shortly after leaving the cell, and 

 became thicker as it approached the nerve-root. When it once entered the 

 root it again became very thin, although now invested with a connective 

 tissue and a medullary sheath. A large number of connective- tissue nuclei, 

 nerve-sheath nuclei, and some unknown elements cause the motor-root of 

 the nerve to swell to some size just after leaving the cord. It decreases 

 again in size before it enters the foramen and leaves the vertebral canal in 

 company with the much smaller dorsal root. 



Just outside the canal the dorsal root ends in the spinal ganglion, and 

 the motor-root traverses the inner side of the ganglion, from which it emerges 

 on the lower edge, and at once divides into a dorsal and a ventral branch. 



The ventral branch passes backward and downward (see text-figs. 8 

 and 9) to a point at a level with the lower middle spindle, and here it gives 

 off a considerable group of fibers which pass caudad just inside of the 

 spindle capsule. These fibers are joined by similar groups from others of 

 the spinal nerves and the whole mass forms the lower middle electric nerve 

 (text-fig. 8, N3). At the posterior level of each electroplax this nerve gives 

 off a few fibers (text-fig. 8, E) which branch out and innervate the posterior 

 surface of this electroplax. 



