188 GEORGE E. NICHOLLS 
however, the fiber may have been broken by an accident at a 
point very far forward it may be supposed that a relatively 
‘long period may elapse before the new growth has pushed back 
to the terminal sinus. 
But regeneration might, under such circumstances, take place 
much more quickly when (unlike the condition after the experi- 
ment) the walls of the central nervous system had remained 
intact and there was no need for the production of a new terminal 
sinus. 
Specimens 38 and 41 are instances in which regeneration must 
certainly have occurred, both of these rays having suffered the 
loss of the hinder part of the tail (and therefore of the sinus 
terminalis) earlier in life. | 
VIII. SUMMARY 
1. Reissner’s fiber, if severed, will generally be withdrawn 
in both directions from the lesion, the retraction being apparently 
effected by a spiral winding of the fiber which attains a greatly 
increased thickness as the withdrawal proceeds. 
2. In dead or dying material, this retraction may continue, 
if not checked by prompt fixation, until the whole of the fiber 
has withdrawn to its points of attachments; in living specimens 
there may be produced at the broken ends a tangle or snarl 
which doubtless serves to prevent such extensive retraction. 
3. In individual rays or dogfish in which such retraction of 
the fiber has taken place there is manifested a distinctive re- 
action; the specimen assumes an abnormal posture while at rest 
and probably, also, exhibits an unusual action while in motion. 
4. This reaction becomes apparent very shortly after the 
return to consciousness (of the animal anaesthetized for the 
operation), may be intermittent, and is manifested by different 
specimens for widely different periods. Probably there is a 
connection between the degree of the reaction and the extent 
of the retraction of Reissner’s fiber. 
5. This reaction is not observed in those individuals in which 
the fiber has been broken but has, for any reason, failed to 
retract. 
