Origin of Electric Tissues of Gymnarchus Niloticus. 



163 



Fig. 3. — Section through mid-tail region 

 of body of an adult Gymnarchus, 

 showing position of the eight electric 

 spindles and their relations to sur- 

 rounding muscular and bony struc- 

 tures. D, dorsal spindles; U.M., 

 upper median spindles; L.M., lower 

 median spindles; V, ventral spindles. 

 (After Fritsch.) X unknown. 



(about 42 days) used for this paper represent practically adult material 

 so far as the histology of the electroplaxes is concerned. A short resume 

 of the adult anatomy will make a good basis for the embryological descrip- 

 tions to follow. 



The fish (see text-fig. i) is a mormyrid of elongate form, so much so as to 

 make it almost eel-shaped, although not quite so much so as the Gymnotus 

 of South America. It possesses an extensive development of the dorsal 

 fin, which extends from forward on the neck to within a short distance 

 of the tip of the tail, where it suddenly stops, leaving the tip of the tail 

 naked of fin; whence the name of the fish, Gymnarchus niloticus. This 

 heavy fin, well provided with a 

 series of lateral ray-muscles, is 

 used extensively by the fish as a 

 means of propulsion, by holding 

 the body straight and stiff and 

 causing a series of lateral undu- 

 lations to pass from behind for- 

 ward, thus driving the body back- 

 ward (see text-fig. 2) or, from front 

 to rear, which causes the fish to 

 move with its head forward. I 

 have not heard, but I presume 

 that in moments of unusual effort 

 the animal can swim by means of 

 the common sinuous body-move- 

 ments used by other elongate fishes, 

 as is well illustrated in the eel. 



The posterior tip of the creature's body is interesting. As has been 

 mentioned, this end is free from the fin for some distance (see text-fig. 2). 

 Also it is round in section and ends bluntly. When swimming backwards 

 the animal uses it like a finger to feel its way. The peculiar round and 

 blunt end may be explained by the fact that this tip contains the largest and 

 best-developed portion of the electric organ, which fills the lateral parts of 

 the body at this point almost to the exclusion of the ordinary muscle. 



As Erdl and Fritsch have described, the electric organ consists of eight 

 long "tube-like" or cylindrical structures, four on each side, embedded in 

 the muscle tissue as close to the median bony parts as a little connective 

 tissue in between will permit. Four of these are present on each side 

 (see text-figs. 3 and 4), and they may be called in order from above down- 

 ward, the dorsal, the tipper middle, the lower middle, and the ventral cylinders, 

 or spindles, of the electric organ. In a section cut through the body at a 

 point midway between tail-point and anus (see text-fig. 3) the dorsal spindles 

 are to be found, just above the union of the neural spines of the vertebrae, 

 and set closely together with only the dorsal spine and some connective tissue 

 between them. The upper median spindles are more widely separated by 



