i82 Papers from the Marine Biological Laboratory at Tortugas. 



fixed in sublimate-acetic, much could be accurately made out, and it is 

 hoped, too, that the description will prove suggestive. 



The motor cells were first looked for in the spinal cord, especially that 

 region which was posterior and in the neighborhood of the electric organs. 

 Assuming for the present, as a law, that electric-motor cells are larger than 

 muscle-motor cells which innervate an equal weight of muscle, it was very 

 easy to find groups of large, heavy nerve-cells scattered through the sub- 

 stance of the spinal cord from near the anterior beginning of the spindles 

 to the very last point in the tail to which they extend. 



.^^=^^==!~ 



Fig. 8. Side view of reconstruction (semi- 

 diagrammatic) of spinal cord and motor 

 electric nerves of a larval Gytmiarchus 

 42 days old. Arrow indicates anterior 

 and posterior directions. A'l, A'^2, N3, 

 and Nt are the four electric nerves formed 

 by branches from motor roots of spinal 

 nerves. E indicates small branches from 

 electric nerves that innervate posterior 

 surfaces of electroplaxes. g marks spinal 

 ganglion, whose afferent nerves have 

 been cut off. 



These cells were situated just above the central canal and from i to 4, 

 or even 5, could be seen in every transverse section in most of this length 

 (fig. 14, plate ^, A, B, and C). They did not appear to be divided into two 

 symmetrical groups, but rather to lie in one median group. On the one 

 hand, their dorsal position might appear to be evidence that they were not 

 motor-cells or that they were not to be considered as modified muscle-motor 

 cells. On the other hand, the presence of real ventral muscle-motor cells 

 (fig. 14, plate 4, D and E) in the whole length of the cord, the correspondence 

 of the cells under discussion with the position of the electric spindles, and 

 the fact that Fritsch describes similarly placed cells in the same position 

 in mormyrids as the motor-nerve cells of the electric organ, all seemed to 

 constitute very strong indirect evidence that these were the nerve cells 

 which innervated the posterior ends of the electroplaxes. 



In addition, the writer was able to trace, in a number of cases, the course 

 of the neuraxons through the cord, out into the ventral roots of the nerves, 

 and finally into special bundles of nerve fibers that undoubtedly innervated 

 the electric tissue. No one nerve process was actually traced for the whole 

 distance unbroken, but the various parts and regions were so pieced together 

 that the course was well established and corresponds in many ways with 

 Fritsch's observations on Mormyrus. Text-figure 9 shows a semidiagram- 

 matic cross-section of these courses as seen from in front, while text-figure 8 

 shows the same thing sketched in relief from the side. This topography 

 will presently be described. 



