386 Journal of Coniparatwc Neurology and Psychology. 



tance from the point of injury. Fig. 5 shows that most of these 

 fibers are in the last stages of degeneration. They are very irreg- 

 ular in shape, and their outlines are only faintly visible. They 

 stain faintly excejDt in a few places, where there still remain 

 minute globules of myelin. They are broken up into short 

 segments, so that no fiber can be followed for any considerable 

 distance. A few droplets of free myelin are still unabsorbed. 



In order to interpret these results it is necessary to bear in 

 mind the condition of medullation in the corpus callosum of the 

 young rat. Watson (7) has shown that medullation in this re- 

 gion begins about the fourteenth day. Thus, fibers cut in the 

 operation performed on the twenty-first day arc both structur- 

 ally and functionally very immature ; and the rapidity with which 

 they degenerate is, no doubt, closely related to this condition. 

 By the seventieth day the neurones have attained a greater de- 

 gree of stability, which shows itself in the much more limited 

 retrograde degenerati(Mi. When the operation was made on or 

 before the seventh day of age no medullated fibers were cut, 

 because there were none present. Ihis would account for the 

 fact that in examining the preparations of these brains no fibers 

 with disintegrating myelin sheaths were found. There are, how- 

 ever, areas which are almost devoid of fibers, although they ex- 

 hibit a few that are slender, normally contoured and well stained. 

 On the basis of the earlier experiments (3) these are to be ex- 

 plained as having developed since the injury. Complete de- 

 generation is best seen in the corpus callosum of the rat op- 

 erated on when twelve hours old, which has been figured 

 in \\\G: Journal of Couil^arativc bJeurology, Vol. XIII, Plate VII, 

 Figures 4 and 7. Much the same condition is seen in the cor- 

 pus callosum of the next older rat (3 days); but in this and the 

 one operated on at the seventh day the wound is situated so far 

 posteriorly, that the picture is complicated by the presence of 

 fibers curving backward into the occipital lobe. These fibers, 

 coming from in front and passing backward and outward, cross 

 the zone which would otherwise be free from fibers. 



As has been already stated, this paucity of medullated 

 fibers in the corpus callosum, so evident in the youngest rat. 



