DEGENERATION AND REGENERATION OF NERVE FIBERS 525 
branches given off from the axons at a later date and at a higher 
level. When these branches instead of growing in a peripheral 
direction, coil spirally beneath the neurilemma or turn back- 
ward in a central direction, they produce the tangled skeins. 
These skeins are seen in the process of formation on the twenty- 
fifth day at a time when all of the tubular networks have dis- 
appeared and are often seen farther up the nerve than the level 
at which the tubular networks appeared. The fact that they 
are more abundant, when the union of the cut ends is prevented, 
is readily explained by the less potent attraction exerted on 
the growing branches by the distal stump, but would not be 
explained by Cajal’s hypothesis. 
Cajal is of the opinion that the normal mode of regeneration 
of the medullated axons consists of the three following stages: 
(1) degeneration of a short stretch of the axon near the cut sur- 
face; (2) the formation of a bulb on the axon above this stretch;: 
and (3) growth of the axon out of the sheath into the scar where 
branching occurs. This sequence of events occurred in few, 
if any, of the fibers in our sections. In our preparations the 
essential factor in the regeneration of the medullated axons of: 
the proximal stump was the formation of lateral branches at a; 
distance of 1 to 5 mm. above the cut surface and within the old: 
sheaths, within which they grew peripherally until they reached. 
the scar. The fact that Cajal worked chiefly with young dogs 
and rabbits, while this work has been done on adult dogs is 
sufficient to explain this difference in the results. 
The recent statements of Dominici (’11) that no new axons: 
can be observed growing out of the proximal stump during the 
first month are not supported by an account of detailed obser- 
vations, and his negative results are clearly due to an inade- 
quate technique. 
MECHANISM OF THE REGENERATION OF NERVE FIBERS 
We havé been concerned in the preceding sections of this 
paper with a variety of changes occurring in the proximal and 
distal stumps, many of which, so far as we can see, take no 
