No. 6.] 



THE LIMBS OF PLETHEDOX. 



291 



In the first of the series of transverse sections changes in 

 the cut muscles are already noticeable, the most striking being 

 the increase in the number of nuclei, especially in the outer 

 fibers of the limb. This increase can be seen in Region I as 

 far up as the origin of the muscles at the elbow. In the outer 

 fibers the muscle tissue is becoming thinner and disappearing, 

 and while the outlines of fibers and bundles are not lost, they 

 are much less clear than in the normal limb. The inner 

 bundles, however, are but little affected, and extend unbroken 

 to the cut end, which is at this time not yet entirely covered 

 by ectoderm. No mitosis is seen in this section. 



In stage 2 the changes are more marked. There has been 

 a continued increase in the number of nuclei in Regions I and 

 II, the outlines of the outer fibers and bundles are lost, while 

 the muscle substance has disappeared except for disintegrating 

 fragments here and there, contrasting sharply with the thin 

 cytoplasm of the neighboring cells. The inner fibers still extend 

 to the end of the limb, which is now covered entirely by several 

 layers of ectoderm. In the neighborhood of the cut three or 

 four mitotic figures are to be found. In the third stage Region 

 III begins to appear as a small knob of undifferentiated tissue 

 behind the cap of ectoderm. In this knob 

 and for a short distance above it among the 

 outer cells karyokinesis is not uncommon. 



If we compare the following stage (4) with 

 the normal limb, the principal changes that 

 have taken place will be very clearly brought 

 out. In Region I the increase in number of 

 nuclei is very great. Even as far as the elbow 

 two to four nuclei may be found in a section 

 of a single fiber, often crowded together so as 

 almost to fill it (Fig. i). Many nuclei are also 

 scattered between the fibers. Below the elbow 

 the number of nuclei increases, the outlines 

 of the outer fibers are completely lost, and 

 the outer half of the limb, which is normally 

 solid muscle, is seen to be made up of a dense mass of nuclei 

 surrounded by loose protoplasmic substance, with here and there 



FIG. i. 



