132 GEORGE FRED SUTHERLAND. 



deeply-staining nuclei at the cut edge. Is this amitosis or 

 fragmentation? Do the daughter nuclei form normal nuclei, or 

 do they divide several times and degenerate? There is no de- 

 finite evidence that nuclei which divide directly ever become 

 normal again. But at successive stages the deeply-staining 

 nuclei become smaller and smaller down to fragments, so that 

 the direct division is probably a fragmentation as a part of the 

 degeneration of injured nuclei. 



Mitotic divisions can easily be distinguished by the formation 

 of chromosomes. All the preparations were examined and the 

 distance of each mitotic division from the cut edge was measured. 

 The results are shown in Table II. In the sections of the normal 

 tail the number of divisions is the smallest, but since up to three 

 days the mitoses are scattered and the number of individuals 

 small, there is no reason for considering these mitoses anything 

 but normal. During the period of degenerating nuclei, there are 

 almost no mitotic divisions close to the edge. On the third day, 

 the nuclei just in front of the cut edge are proliferating rapidly; 

 at four days there are a few divisions past the cut; at six days 

 there are almost as many divisions in the new spinal cord as in 

 the old; at eight and nine days most of the divisions are in the 

 new cord ; at fourteen days there are scattered mitoses only, both 

 in the old and new cord, and at sixteen days most of the dividing 

 cells are in the very end of the new cord. If later divisions follow 

 this general trend, it seems likely that the rest of the spinal cord 

 will be formed by a growing zone at the tip, and until the new 

 cord is complete the number of mitoses near the tip would prob- 

 ably decrease gradually. 



Fig. 12 gives the average number of mitoses in the spinal cord 

 at each stage, and therefore represents the rate of growth at these 

 times. On the second day there is a considerable mass of tissue 

 over the whole wound, though only degenerative changes have 

 been taking place in the nerve cord. Beginning about this time, 

 the nuclei in the end of the cord loosen and draw apart some- 

 what, stretching out the cytoplasm between them (Fig. ii). 

 This is apparently the first extension in length of the spinal cord. 

 At three days active proliferation of cells has begun but the 

 pulling apart or stretching toward the cut edge continues. Fig. 



