Origin of Electric Tissues of Gymnarchus Niloticus. 169 



separated from the rest of the myotome. Further, there are to be seen on 

 each side three other similar sections of the same muscle-like tissue, all 

 more or less also separated from the main mass of the myotome. The most 

 important part of our study now consists on the one hand in proving that 

 this mass L.M., in figure 5, plate 2, is derived from the undoubted muscle- 

 fibers (marked with O), as seen in figure i, plate i, and on the other hand in 

 showing that this same structure is to become the finished electric tissue as 

 seen in such advanced development as in figure 21, plate 8, for instance. 



In tracing it back to the muscle, we are much assisted by the fact that 

 the dorsal and ventral spindles are always in an earlier or in a less complete 

 stage of development than the upper middle spindle, or, particularly, the 

 lower middle spindle under consideration. And, since in this embryo of 12 

 days the development has gone to considerable length, and since a slightly 

 younger stage, say a 10- or ii-day embryo, was not included among the 

 embryos at my disposal, this fact is of much importance, because it will be 

 fair to take the left ventral spindle as an intermediate step in the compari- 

 son. Figure 9, plate 3, is a highly magnified section of the locality of the 

 left ventral spindle from region C in the embryo of 12 days, and in it we can 

 see a mass of muscle-like cells, closely associated and lying at the inner 

 edge of the myotome. Certain changes clearly differentiate them from the 

 rest of the muscle, however. One change is the fact that the myofibrils 

 have shown a large diminution in size or thickness and have also suffered in 

 power to take the stain; particularly on the periphery of some of the muscle- 

 columns they almost refuse to take it. Their spacing is also irregular and 

 they show a distinct tendency to clumping together and, in some cells, to 

 get close to the nucleus or even to surround it. They can be readily com- 

 pared in figure 9, plate 3, with the well-developed young muscle cells just 

 outside and to the left of them. The dotted line marked XX indicates a 

 separation of the two. All of those to the right of this line show the con- 

 dition of the myofibrils mentioned above; those to the left show the usual 

 condition of muscle cells of this age in fishes. 



A second characteristic of the changing muscle cells under discussion 

 is in their cytoplasm. It appears more abundant, although this may be 

 due to the smaller fibril bundles. But it also stains more heavily with 

 such stains as eosin, erythrosin, and orange G. With the eosin, for example, 

 it also shows a more yellowish tinge than the cytoplasm of the usual muscle 

 cells in the same sections. And lastly, some of the muscle cells seem to 

 have entirely disappeared or to have greatly shrunken. This latter fact 

 causes a loose and separated condition to obtain among the metamorphosing 

 cells which shows in sharp contrast (fig. 9, plate 3) to the compact con- 

 dition seen in the typical muscle cells to the left of or outside of the line XX. 



Another important fact can be seen among the changing cells in figure 

 9. Those near the center of the group show a tendency to touch or 

 coalesce with each other. Already at this early stage this marks a differ- 

 ence in the group. Those cells within the dotted circle are destined to 



