186 GEORGE E. NICHOLLS 
In young animals the recuperative powers are frequen ‘ly much 
greater than in aged specimens and it may prove that the 
restoration of the normal (functional) condition of Reissner’s 
fiber after injury may be effected much more quickly in some 
specimens than in others. 
In this connection the condition of Reissner’s fiber in the 
dogfish (F) is of interest. This specimen, it will be remembered, 
was one which was seen in the large aquarium tank exhibiting, 
very markedly, the reaction which is associated with the broken 
and retracted fiber. The fish had certainly been in confine- 
ment for some time and there was reason for connecting the 
injury to the hinder border of the caudal fin with damage in- 
flicted by the trawl. The injury was, therefore, probably of 
long standing and the reaction had almost certainly persisted 
for a considerable time. In the sections the fiber was found to 
be somewhat slack, lying near its free end in loose undulations 
and there were no indications that regenerative processes were 
at work. The specimen was unusually large and presumably 
an old individual. There seems, therefore, to be in this case 
a connection between the size (age) of the specimen, the lack of 
regenerative powers and the continuance of the reaction. 
As already noted, I have but one example of undoubted re- 
pair of the mechanism (fig. 31) after the experiment. This is 
seen in a dogfish (2) which was killed sixteen days after the 
operation. 
In several cases, however, the fiber had clearly undergone a 
considerable retraction but had straightened out again before 
the specimens were killed. In the sections, therefore, it is seen 
to extend almost or quite to the spot where it had been broken 
by the experimental incision. Examples of this phase of repair 
are to be seen in nos. 5, 7, 8 and 24. The disentanglement 
of the snarl (10) and the unwinding of the spirally twisted 
fiber (37, 41) must be regarded as preliminary stages in the proc- 
ess of repair. In no. 18, where there had been no retraction, 
it would seem as though the end of the fiber was flaring out in 
preparation for a new terminal plug (fig. 28). 
