Origin of Electric Tissties of Gymnarchus Niloliciis. 167 



myotome the smallest cells are seen with large nuclei and no myofibrils at all 

 (fig. 2, plate I, C). These youngest cells are particularly abundant at the 

 dorsal and ventral edges of the myotome. 



From what we know of the position of the future electroplaxes and their 

 relation to the muscle-masses, we can be sure that it is from the inner edge 

 of the myotome that the electric tissue is to come, and a close scrutiny of 

 the cells which form this edge shows that at two points only is there any 

 indication of such a development. 



One of these points, marked with a circle ( ) in figure i, plate i, is where 

 the two myotome segments (dorsal and ventral) meet and close in against 

 the notochord. Here we see, in some of the sections, several especially 

 large and strongly developed fibers, somewhat detached from the rest of 

 the myotome and resting against the vertebral disk. Several reasons exist, 

 however, why these fibers do not represent the future electric organ. First, 

 they are in the exact median position which remains constant during growth 

 and in which no electric tissue is to appear. Second, they are very short 

 and are not attached to each other longitudinally, as other fibers are, by 

 means of connective tissue, but are attached to the bodies of the future 

 vertebra. They may be called the vertebral fibers. 



A second point can be seen where some muscle-tissue shows unusual 

 development. At this point (marked with a circle (O) in fig. i, plate i) are 

 several muscle cells which show from 10 to 18 myofibrils each, and a degree 

 of development almost equal to the cells in the outer layer. These cells 

 are represented as seen under high magnification in figure 3, plate i. They 

 are, I believe, destined to be the future electric cells, several of which will 

 unite and form one of the electroplaxes of the lower median spindle of the 

 electric organ. I base this assertion only on their position and their some- 

 what advanced development as muscle fibers, for they show at this time no 

 indication, other than their size, of developing into electric tissue. 



Their position is slightly too far dorsal for the lower spindle, in an adult 

 Gymnarchus, but when we examine the 12-day-old and 42-day-old stages, 

 we find that the normal growth of the muscle-mass will carry this point to 

 exactly the proper position for that spindle to lie at, in the grown fish. 



An important point in studying these cells in figure 3, plate i, is to note 

 that they are separated from the rest of the myotome and from one another 

 by other muscle cells of weak or of earlier development and containing only 

 a few myofibrils. These weaker cells, and even some of the more advanced 

 ones, are destined to degenerate during the development and growth of 

 the electroplax. In figure 3, plate i, 7 cells are present that will probably 

 take part in the formation of the electric spindle at this point. Some of 

 them are contiguous and others are widely separated. 



In figure 4, plate i, we have a fortunate longitudinal section from the 

 adjacent region of this same embryo. The section shows in longitudinal 

 view, and under low magnification, the same group of muscle cells marked 

 with the circle (O) at both ends. Also, it shows a portion of the ventral 



