170 Papers from the Marine Biological Laboratory at Tortugas. 



unite with one another in a compact bundle to form the future electroplax; 

 while those of the group which lie outside of the dotted ring are to degenerate 

 and atrophy altogether, in order to make room for the growing electroplax. 

 Just mediad of, or to the right of, the dotted ring is seen a cell that has 

 entirely lost its myofibrils and whose cytoplasm is vacuolated and about to 

 be absorbed. At other points, to the left, still other cells can be seen that 

 have dwindled to a smaller size than any of the normal muscle cells. The 

 cells within the ring are judged by the writer to be those which will form 

 the electroplax at this point, because they are starting to unite and, also, 

 because they occupy the position at which the electroplax will lie. Another 

 reason is that their myofibrils are weakening and clumping. Some of those 

 cells lying inside of the ring may also atrophy. This can not be infallibly 

 judged; but certainly all of those outside of it and to the right of the line 

 XX are about to degenerate and are not found in later stages. 



The connective tissue creeps into the neighborhood of and among these 

 metamorphosing muscle cells at this time, and good mitotic figures can be 

 seen, showing that it is increasing the number of its cells. It does not, 

 however, penetrate the groups of future electric cells inside the dotted line. 

 Also blood, pigment, etc., are to be seen in characteristic positions. 



A transverse section of the body at region Bi need not be illustrated at 

 this point by a low magnification figure, because it is so like figure 5, plate 2, 

 in general appearance. But it happens that in such a section several very 

 interesting stages, forming a sequence of which figure 9, plate 3, can be taken 

 as the first member, were noticed, and figure 10, plate 3, is the second in 

 this series. This drawing represents the right ventral spindle in region Bi, 

 and a number, five to be exact, of the transforming cells can be seen here in 

 closer union than the corresponding cells were in figure 9, plate 3. One or 

 possibly two of these may disintegrate a little later. A dotted ring is not 

 necessary, because the connective-tissue cells have partly marked oflf the 

 electric cells, and outside of this incomplete ring and above it can be seen 

 two muscle cells which are atrophying. A third cell is shown in a final 

 stage of disintegration. Its cytoplasm is almost clear, or all gone, and its 

 myofibrils have united in a single lump, which will soon become a round 

 droplet or cell-inclusion that will afterwards disappear. 



Moving to the right upper middle spindle (fig. 11, plate 3), we need very 

 little explanation to see how the four or more muscle cells that first com- 

 posed this structure have come into a still closer union. Below and to the 

 right (outer) are still seen some of the degenerating muscle cells — five in 

 this figure. 



Several important points must be discussed in connection with this 

 figure; the component cells, as seen here, are not simple, single muscle 

 fibers. The lower one can easily be seen to have two myofibril bundles as 

 well as two nuclei. The presence of two widely separated fibril bundles, 

 as well as the large size of the cell, makes it nearly certain that the structure 

 was formed by the coalescence of two muscle fibers. In the upper region 



