168 ADA R. HALL 
of these cells inward through the plug toward the nerve cord. 
We may conclude therefore that the epithelium does not give 
rise to the cicatrix cells nor to the regenerating nerve cord, but 
merely covers over the exposed area formed by the cut. 
There are two muscle layers: the circulars, lying just beneath 
the epithelium, forming a band around each metamere (fig. 1, 
C. M.), and the longitudinals, which extend lengthwise, forming 
a muscular cylinder just inside the circulars (ZL. M.). As stated 
before, the first reaction of the muscles when a cut is made is 
the contraction of the circulars. This tends to draw the cut 
edges of epithelium together and also holds the. longitudinal 
muscle ends toward each other. The formation of the plug is 
very rapid; at first it merely fills the spaces, showing no direct 
connection with the muscles. It is easily seen that these cells 
are not proliferations of the muscle layers. As regeneration © 
progresses, the plug cells elongate and orient themselves with 
their long axis in the direction of the muscle pull. 
In figure 15 longitudinal fibers are seen between the cut ends 
of the longitudinal muscles (L. M. F.), and between these and © 
the epithelium is an area of cut ends (C. M. F.), showing that 
the circulars are also connected by these fibers. Friedlander 
sugcests that the plug cells come from the muscles, and from the 
shape of the nuclei after a day or two of regeneration one might 
be led to think so. However, from the rapidity of formation 
and the appearance of the cells in the earliest stages this is not ~ 
probable. At a very early stage (two and a half hours, fig. 3), 
the plug is formed and closely packed with round nuclei, but 
there is no particular direction to the fibers. At ten hours 
(fig. 12) the nuclei are still large and round. Only in the later 
stages do the nuclei elongate and the fibers show a definite 
orientation. I believe that this is a mechanical effect, in part 
at least, due to the pull of the old muscles after a definite amal- 
gamation has taken place between their raw edges and the cells of 
the plug. Figure 6 shows the close relationship between the 
plug and the muscle fibers. 
Heschler believes that the subsequent growth and regenera- 
tion comes as a proliferation of the old muscle cells or of undif- 
