172 Papers from the Marine Biological Laboratory at Tortugas. 



shown, as well as their relations to the rest of the myotomes. Bracket 

 No. I (zone i) in this figure indicates the inner tips of the ventral halves of 

 the myotomes. Zone 2 shows a mass of connective tissue and nerve which 

 runs between these two halves (compare fig. 5, plate 2, Conn T.). Zone 3 

 embraces the inner tips of the dorsal myotomes where they lie between 

 the upper median spindle and the horizontal connective-tissue septum. 

 It will be noticed that this set of muscle fibers appears to be very short, 

 much shorter than the connective-tissue regions between them (not filled 

 out in this drawing). The reason can be seen at once when these same 

 muscle cells are examined under the high power (fig. 15, plate 5, bracket 3; 

 and fig. 6, plate 2), where it is apparent that they are degenerating fibers. 

 Parts of four of these fibers are shown in figure 15, plate 5, and that one 

 nearest the electric tissue which is represented in figure 15, plate 5 (brackets 

 4 and 4i), is the farthest gone, having lost its myofibrils altogether. The 

 nuclei are also distorted and, besides the single large nucleolus, are noticeably 

 empty of any chromatic matter or even of linin (notice the nuclei of healthy 

 muscle and electric tissue). The next fibers to the left in this figure serve 

 well to show how the degeneration takes place from the ends, where the 

 cytoplasm is gathered in heavy lumps which stain light and yellowish with 

 eosin. In fact, there is a singular similarity between the process of degen- 

 eration of these muscle cells and the transformation of the others into the 

 electric tissue. Above, in zone 3, is a rounded cell which I take to be a 

 muscle cell in a final stage of dissolution. It contains large granules of a 

 chromatic substance, which appear to be the remains of myofibrils. Figure 

 6, plate 2, also represents three fibers from this same region in another 

 myotome and shows three stages of the degeneration of the muscle. Two 

 of them indicate that the degeneration begins in the middle of the fiber. 



Zones 4 and 5 in figure 15, plate 5, as well as in figure 12, plate 4, show the 

 young electric tissue of the upper median spindle. Let us first consider this 

 tissue in figure 12, plate 4, where we can get a larger, low-powered view of 

 it. In the first place, the most noticeable feature of the electric tissue here 

 is that it shows scarcely any signs of a division into the original myotomes. 

 A trace of this is seen in zone 4, but in a careful and systematic search 

 through a number of the spindles of this stage very few instances could be 

 noted. Even the myofibril bundles of successive myotomes seem to have 

 united, and these bundles were most carefully examined under a Zeiss 2 mm. 

 1.40 ap. lens. I have no doubt that each spindle does constitute a con- 

 tinuous reticulum of muscle cells at this period. Laterally, the various 

 muscle cells are not continuously united, but, as is shown in figure 15, plate 5, 

 they are united at a number of points much in the way that some heart- 

 muscle fibers are. 



The myofibrils are best seen, of course, in these longitudinal sections, 

 and it can be seen that they are typical. The transverse striation is as 

 perfect as in the functional muscle, but owing to the slighter fibrils they are 

 not quite so dark. In the upper median spindle of figure 15, plate 5, some 



