202 JoURNAL OF COMPARATIVE NEUROLOGY. 
these cells would be sufficiently rapid to explain the decrease 
in the number of the crossing fibers. An observation, which 
tells strongly against the regeneration theory in this case, is. 
that the fibers of the corpus callosum degenerated toward their 
cells of origin after being cut in the two younger rats, while 
they remained intact on the cell side of the injury in older rats. 
This observation shows that nerve fibers instead of possessing 
unusual regenerative power in these young animals (where the 
largest number of nerve fibers were found in the scar) tend to 
degenerate completely after injury. 
Summary. 
(1). The adhesions which are described by other investi- 
gators as binding the brain scar to the meninges and the men- 
inges to the tissue filling the skull wound were entirely absent 
in these young rats. 
(2). Very little scar tissue was found in the brain. In 
the upper part of the cortex it was not noticeable in Pat- 
WEIGERT preparations; in the substantia alba it was more abun- 
dant. With the exception of the large triangular area in the 
brain of rat No. 4, there is a steady decrease in the amount of 
scar tissue with the decrease in the age of the animal at the 
time of the operation. How little connective tissue there is 
in the wound is seen in Fig. 10. Nerve cells quite normal in 
appearance border directly on the line that represents the path 
of the knife. Only here and there cana connective tissue 
cell be found in the scar (c). Inthe brains of older animals 
the connective tissue is very abundant at the site of the lesion. 
It may be that the absence of a dense connective tissue scar in 
the cases here given may in part explain the positive results 
obtained on the crossing of nerve fibers, but the increase in the 
number of nerve fibers is out of proportion to the decrease in 
the amount of scar tissue. 
(3). A degeneration of the fibers of the corpus callo- 
sum toward their cells of origin was noticed in the two younger 
rats. 
(4). Shifting of the parts of the cortex with reference to 
