430 
and would, moreover, be quite in agreement with many of the ob- 
servations recorded by SARGENT. 
Further, the fibre would appear to exist in life under considerable 
tension and to be as a result somewhat stretched. When released, the 
severed ends were withdrawn into a spirally twisted knot precisely like 
that which would be formed in a fine elastic thread which, attached 
at one end, was continually twisted in one and the same direction 
from the free end. 
This recoil was, in the experiments, occasionally prevented or 
postponed by the pinching together of the walls of the canalis cen- 
tralis in the terminal filament, which thus gripped the free end (or 
ends), but when the recoil did occur the knot which resulted soon 
brought the withdrawing thread to a standstill, and this so suddenly 
that the ependyma was partially disrupted or even the terminal fila- 
ment shattered. 
Where, however, REISSNER’s fibre was broken at a point sufficiently 
near to a widening of the central lumen (as near the fourth ventricle 
in front or the sinus terminalis behind), the retraction seems to have 
proceeded until the fibre brought up sharply at or near its point of 
attachment, from which it would even jerk itself free, or which, in the 
case of the sinus terminalis, it completely demolished by the violence 
of its recoil. 
The constantly spiral recoil indicates the existence of some very 
definite internal structure in the fibre although none can be demon- 
strated by the methods employed. An explanation may perhaps lie 
in the manner of development of the fibre. It appears to arise 
in embryonic or larval life as a number of long, extremely fine, 
cilia-like processes which grow out from the cells of the sub-commis- 
sural organ and unite in the iter to form a single thread. This extends 
backwards through the brain ventricles and enters the canalis centralis 
of the spinal cord. Along the length of this it is found later to be 
stayed and supported by numerous cilia-like processes from the epen- 
dymal cells. It may well be that these, growing backward, continue 
the fibre and are in their turn overlaid by fresh constituent ependymal 
cilia. If the whole of these fibrillae have a slight spiral growth the 
mode of recoil of the complete fibre formed by their union would be 
easily understandable. 
The occasional occurrence of a tangled mass of fibre in the sinus 
terminalis, in addition to the normal taut thread, in specimens in which 
the central nervous system has been preserved whole, can only indicate 
an accidental breakage of Reıssner’s fibre in life. While, however, 
