56 
GEOEGE E. NICHOLES. 
slight knotting and coiling to be observed in the fibre (fig. 13) 
must have existed in life. In specimen D, however, the cord 
and Reissner’s fibre were clearly severed prematurely, before 
the latter had been quite fixed at its posterior end. A certain 
amount of recoil had taken place, so that for about three 
millimetres the canalis immediately behind the point of 
cutting had had the fibre withdrawn from it. Where the 
fibre appears it is found to stretch back in a tolerably straight 
piece almost to the sinus terminalis. About at the point, 
however, where the central canal begins to widen out, just 
anterior to the sinus terminalis, the fibre passes into a 
considerable tangle (fig. 40, r./.'j, from the hinder end of which 
it emerg-es to run in a short straight course into another and 
much larger tangled mass (fig. 40, r.f."). This second tangle 
is that conical heap which forms so conspicuous an object in 
the sinus terminalis (fig. 12, r.f."), the base of the cone 
lying against the postero-veutral wall of the sinus ter- 
minalis. Its apex, into which the straig’ht part of the fibre 
passes, projects dorsally and a little forwards. As a post- 
mortem recoil has here undoubtedly occurred, it is not now 
possible to decide whether any of this tangled mass of the 
fibre was present as such in the living animal. It is my 
opinion, however, that the intricately tangled mass (fig. 40, 
r.f.') lying near the end of the central canal is alone sufficient 
to account entirely for the comparatively small amount of 
retraction from the anterioi- end to which I have referred. 
In addition to this material which I myself prepared I have 
examined a number of series of sections through the tail of 
Petromyzon fluviatilis which are in the collection at 
King’s College. In two of these I found the sinus termi- 
nalis almost intact, but in one only could Reissner’s fibre 
be traced backwards to that point. In that series, which was 
cut horizontally, the fibre may be seen emerging from a 
widely open terminal neural pore as a taut thread, which 
passes at its extremity into a mass of indeterminate tissue 
and is lost. The meningeal walls of the sinus terminalis 
in this specimen are very ill-defined. 
