Origin of Electric Tissues of Gymnarchiis Niloticus. 185 



The remainder of the ventral branch passes farther down and again gives 

 off a branch which goes to form part of the ventral electric nerve (text-fig. 

 8, Ni^. This latter sends off little branches to furnish the posterior surfaces 

 of the ventral electroplaxes with motor-fibers (text-fig. 8, £). 



Going back to the anterior root, we find that its dorsal branch leads 

 directly upward and, passing between the spinal ganglion and neural 

 arch, slopes gently backward to give off a large branch which becomes a 

 part of the upper median electric nerve (text-fig. 8, iVo). Its remaining 

 fibers reach upward and furnish the dorsal spindle with a part of the fibers 

 that form its dorsal electric nerve (text-fig. 8, Ni). 



Of course, there are muscle-motor elements in both these branches of 

 the anterior nerve-root, the number depending upon the position, back- 

 ward or forward, at which we examine the arrangement. Two examples of 

 the motor-nerve cells of the muscle-tissue from the cord in region D are 

 shown in figure 14, plate 4, D and E. At the level of the anterior parts of 

 the electric organ, when there are large quantities of muscle and the electro- 

 plaxes are very small, the muscle branches are large, particularly the dorsal 

 branches, which have to supply the muscle-bundles for the large dorsal fin. 

 In the posterior region of the tail, on the other hand, the muscle is almost 

 entirely absent and the muscle-branches of the anterior roots are not even 

 easily seen. 



The eight (four on each side) longitudinal electric nerves are interest- 

 ing in that they form a morphological buffer between the conflicting 

 segmentation of the nervous, skeletal, and muscular systems, and the inde- 

 pendent segmentation of the electric system. Were the electric segmenta- 

 tion to correspond with that of the others, we should not find these nerves 

 in this recognizable form. In fact, in the anterior part of the electric organ 

 we do not find them as continuous nerves, in many places, for more than 

 two or three neuromeres at a time. And even posteriorly where they do 

 form continuous nerves, the fibers that enter them from any given spinal 

 nerve do not pass very far back in them before leaving to innervate one of 

 the electroplaxes. Each nerve cell probably lies but a very short distance 

 in front of the electroplax which it supplies. This was decided upon by 

 plotting the relative positions of all motor-electric cells and all electroplaxes 

 in the larger part of the organ. 



While doing this it was also determined how many nerve-cells sent 

 their nerve-processes to each electroplax. Thus, in region E of the 42- 

 day-old larva there were easily counted 414 of the electric-motor cells, while 

 in the same part there were 81 electroplaxes. This makes it quite sure 

 that, on an average, about 5 of the nerve-cells were used for each electro- 

 plax. It was attempted to count the nerve-fibers as they left the electric 

 nerve to branch out over the electroplax, but the elements were too small 

 and the fixation not just what was needed to do this. It could be easily 

 done in a grown specimen. 



I shall now take up the structure of the nerve-fibers as they leave the 



