Origin of Electric Tissues of Gymnarchus Niloticus. 173 



points were found where this striation was sHghtly weakened. Cases of 

 relaxation and semi-contraction uniformly characteristic of the normal 

 muscle prevailed. The distances between the bands, as well as the length 

 of the anisotropic parts, was the same as in ordinary muscle and remains 

 as long as striation can be seen. This same fact does not hold true in Raja 

 or in Astroscopus, where it is much shortened. It does hold, however, 

 in Mormyrus, which is closely related to Gymnarchus. 



The longitudinal sections, shown in figure 12, plate 4, and figure 15, plate 

 5, correspond to the spindles before it has become possible to see that they 

 have segmentally divided into electroplaxes and before a central core of 

 fibrous material has become definitely differentiated from a superficial layer 

 of cytoplasm containing all of, or nearly all of, the nuclei. 



We will now advance toward the head of this same specimen, to find 

 material in a more advanced stage of development, in order to study the 

 segmentation of the embryonic spindle into its individual electroplaxes 

 (this segmentation has just been described as absent in fig. 12, plate 4), 

 and to study further the differentiation of the inner fibrous core from the 

 outer layer, and lastly to see how the myofibrils lose their transverse stri- 

 ation (muscle striation). The reader will remember that at this stage the 

 anterior electroplaxes are in advance, developmentally, of the posterior ones. 

 Figure 13, plate 4, shows four vertebral segments, taken from the region C 

 of this same embryo, under low magnification. Bracket i embraces the 

 zone of the epithelium. No connective tissue is shown. Between the 

 muscle-zones 2 and 4 lies the zone of the long, narrow, embryonic electric 

 spindle, marked by bracket 3. It is the lower median spindle which is 

 shown, one which is always in advance of all the others in development. 

 In this figure it becomes quite plain that, while the muscle tissue is arranged 

 segmentally to correspond to the vertebra, the electric organ is not. In 

 this case the electric spindle is separated transversely by connective tissue 

 into three segments, instead of four. Nor is this proportion always main- 

 tained among the several electric spindles themselves. 



In some cases, as will be seen later, a single electroplax corresponds to 

 as many as five vertebrae. Even the electroplaxes do not correspond with 

 each other. A subsequent figure will also demonstrate this (fig. 23, plate 9). 

 Just what factors do determine the length of the electroplax I am unable 

 to say. A closer study of the nerve distribution and blood-supply may 

 throw some light on the matter. 



The transverse sections of the stages represented in figure 13, plate 4, do 

 not show anything of interest over and above what was discussed in con- 

 nection with the conditions seen in figures 9, 10, and 11, plate 3, and figure 7, 

 plate 2. We will, therefore, advance toward the head one step further in 

 this embryo and examine a longitudinal section from the region marked B 

 in text-figure 6. Here (fig. 1 7, plate 6) the electric tissue is seen in what may 

 be considered as its maximum development in this 12-day embryo. The 

 electroplaxes are distinct from each other and have grown considerably in 



