DEGENERATION AND REGENERATION OF NERVE FIBERS 529 
associated with the violent reaction going on in the neurone at 
that time—the so-called ‘axonal reaction.’ 
Unfortunately the eight-day specimen was divided into proxi- 
mal and distal stumps by a cut through the scar near the cen-, 
tral end before the tissue was prepared by the pyridine-silver 
method; and, as a result, the stain of the central portion of the 
scar is not good. On the fourteenth day, however, the scar 
is beautifully stained and fibers can be seen in large numbers 
leaving the central stump, plunging into the scar, and running 
through it in every direction. The same is true of the nine- 
teen-day specimen. Here many fibers are seen working their 
way centrally in the thickened perineurium of the central stump 
in which they form a plexus connected with the plexus in the 
scar. These represent a portion of the aberrant branches that 
never find their way into the distal stump. Where fibers are 
seen to come to an end within the thickness of a section, they 
are tipped with a small bulb. -On the twenty-fifth day (Dog 
x, in which union of the stumps was prevented by excision of 
1 cm. of the nerve) the mass of scar tissue covering the end of 
the proximal stump is penetrated in every direction by these 
fine nerve fibers; from this mass covering the end of the central 
stump, they can be followed in gradually decreasing numbers 
upward in the thickened perineurium for 4 mm. and distal- 
ward in the scar for a distance of 18 mm. As one goes distally 
in the scar the number of fibers gradually decreases. No fibers 
reach the distal stump, and no fibers grow into the scar from 
the distal stump. 
Figure 27 is drawn from the scar in the immediate neighbor- 
hood of the central stump of the specimen just described. It 
will be noticed that the fibers are arranged in bundles. The 
explanation of the formation of these bundles is the stereotro- 
pism demonstrated by W. H. and M. R. Lewis (’12) in growing 
fibers. When the growing tip of one fiber comes in contact 
with another fiber, it follows this second fiber just as, in the 
experiments of the authors just mentioned, growing fibers run 
along the under surface of the cover glass. It will be further 
noted that the bundles, while crossing each other in a more or 
