No. 6.] 



THE LIMBS OF PLETHEDON. 



295 



the first place there is a great increase in the number of nuclei 

 zvit/iin the old fibers, but in no case is any karyokinesis found 

 there. This degenerative process in the old fibers must there- 

 fore take place by direct division of the nuclei. Instances of 

 this division are shown in Figs. 5 and 6. To this division and 

 to the disintegration of some of the old fibers is due the enor- 

 mous accumulation of nuclei in the outer part of the limb 

 (Fig. 2). The cells so formed then begin to divide by karyo- 

 kinesis in the region of the cut, and thus a further increase in 

 their number takes place. In these outer cells new muscle 

 tissue forms and the new fibers are built up. A certain 



FIG. 5. 



FIG. 6. 



number of the old fibers remain in the middle of the limb, 

 and in these the muscle tissue never disintegrates, though 

 it splits longitudinally. 



Again, as at any one level the number of nuclei far exceeds 

 the number of fibers in a normal muscle, a great number of 

 them must, between the early stages and the fully formed limb, 

 either degenerate or be transported (cf. Figs. 3 and 4). That 

 this is so is easily seen, for two reasons: (i) when the new 

 fibers form, at a given level several nuclei are often included 

 in one fiber ; when the limb is full-grown there is only one ; 

 (2) among the newly formed fibers, but between them, are many 

 scattered nuclei ; the majority of these disappear in later stages. 



