426 
very flattened epithelium. It is wholly enclosed within the vertebral 
column, although in the embryo it extends beyond and behind the 
notochord just as it still does in the adult Raja blanda. 
In addition, however, to the part of Reıssner’s fibre which ends 
in the terminal plug, there occurs in the sinus terminalis of this 
specimen a mass of loosely coiled and very swollen fibre the relation 
of which to the terminal plug could not be determined, but its occur- 
rence is only to be explained as in the skate and in Rhina, as the 
result of some breakage of the fibre during the life-time of the animal. 
The cutting of the fibre in dead material, before fixation, thus 
results in the formation of a tangle due to an extensive spiral retraction 
which appears to continue until the fibre has withdrawn right to its 
point of attachment, unless, as in the case of the specimen (C) of 
Rhina, above referred to, the tangled mass reaches such a size as to 
block up the canalis centralis and so to prevent further recoil. In 
such a case the snarl, in its recoil, may exert sufficient force to rupture 
the walls of the terminal filament, indicating that the tension of the 
unbroken fibre must be very high. 
b) Material used in the Experiments. 
We pass now to a consideration of the results of the experiments 
as revealed by a study of the material by means of serial sections. 
It will be remembered that in all cases the operation consisted simply 
in making a minute incision close to the extremity of the filum terminale 
with a view to severing REISSNER’s fibre, and also that in the majority 
there appears to have been time for the partial restoration of the 
fibre to its usual condition before the specimens were killed. 
Two of the series of sections which were cut (6, 11) are not 
sufficiently satisfactory to make it safe to base any deductions upon 
them. The same is also true, to a certain extent, of (9), in which, 
although an interesting spiral coiling of the fibre was found (Pl. I, 
Fig. 6) in front of the point of section, this condition was due almost 
certainly to postmortem injury to the fibre. A similar explanation 
may account for the extraordinary spirally coiled condition of the 
fibre in the specimen (3) of Raja blanda (Pls. I, II, Figs. 7, 8, 9) but of 
course even if this is so, in neither of these cases will the condition 
of the fibre have been affected behind the point where the experimental 
incision was made. 
Leaving the doubtful cases out of consideration for the present 
and confining our attention to the remaining experiments it will be 
evident that these fall naturally into three groups. 
