No. 6.] 



THE LIMBS OF P LETHE DON. 



293 



fibers, often with several nuclei, and broken frequently into 

 quite small fragments. Outside these are numerous nuclei, as 

 before, but now surrounded by very distinct muscle tissue 

 (Fig. 3). This tissue has formed often about several nuclei 

 in a group, and has no distinct walls ; there seem to be as yet 

 no distinct muscle fibers. But the line of separation between 

 the old muscle fibers and the new tissue is distinct, and a few 

 old fibers can be traced to the region of the cut, although at 

 that level the greatest 

 part of the tissue is 

 new. New muscle sub- 

 stance has appeared 

 about the nuclei for a 

 short distance below the 

 cut ; but it decreases as 

 we pass down, until we 

 find only crowded nuclei 

 and thin protoplasm. In 

 this region, as before, 

 numerous karyokineses 

 are seen. 



In Region I of the 

 sixth stage all the mus- 

 cle fibers are small and 

 the definite line between 

 the old and new is lost. 

 The majority of the 

 fibers contain in cross-section but one nucleus, though some 

 may contain two or three, and in general the smallest fibers 

 and most nuclei are on the outer side of the limb. This is espe- 

 cially noticeable in Region II, where the outer (new) fibers are 

 exceedingly small. The muscle tissue decreases in amount as 

 we pass to Region III, until it is all lost. Cells dividing by 

 karyokinesis appear at this level. 



Stage 7, though of the same age as the preceding, is 

 somewhat further differentiated, and in this the new fibers 

 are more rounded and have assumed a more characteristic form 

 (Fig. 4). A comparison with the normal shows smaller fibers 



FIG. 3. 



