411 
winding of the fibre. The knot formed thus has precisely the character 
of a tangled heap such of as would result from the continued twisting 
in one direction of one end of a thin elastic thread of which the 
other end was held fast. Several intermediate stages between the 
simple spiral and the complicated twisted knot are figured in actual 
photomicrographs of the fibre in the skate (Pls. I, II, Fig. 7, 8, 9 R.f.). 
In every such instance there is a marked decrease in the length and 
an accompanying and very considerable increase in the diameter of 
the fibre. 
I have also shown (1909) that the tension under which the fibre 
normally exists is such that if the fibre in a freshly killed animal be 
subjected to an unusual strain it may snap and the free ends will 
then recoil with considerable violence. The case of a specimen of the 
common toad was instanced where the fibre had sprung forward with 
such force as to actually drive through the thin roof of the fore brain. 
That an accidental snapping of the fibre may occur in life I have 
since had considerable evidence to show, and I shall later in this paper 
have occasion several times to refer to certain appearances in the 
terminal sinus of different elasmobranchs which may almost certainly 
be explained only in this way. 
REISSNER’s fibre arises in the brain beneath the posterior com- 
missure, from that tract of modified ependyma for which the name 
sub-commissural organ was recently suggested (Denby & NICHOLLS, 
1910). From the ventricular surfaces of the cells of this sub-commis- 
sural organ a number of fine long cilia-like processes grow out and 
coalesce to form a single thread which may be followed backwards 
through the brain ventricles. It is strictly median in position and as 
it passes from the iter it lies closely against the ventral surface of 
the rhombo-mesencephalic fold, upon which it may apparently score a 
deep groove. It enters the canalis centralis of the spinal cord, through 
which it passes, receiving frequently in this part of its course other 
cilia-like processes from the ependyma lining the central canal. Finally 
it emerges from the central canal, through what must be considered 
as a terminal neural foramen into the sinus terminalis (Pls. I, II, Fig. 2, 
4.5 si.) 
SARGENT (1901, 1904) supposed Retssner’s fibre to consist of a 
number of nerve fibres (axons of the ‘Dachkern’ cells), which were said 
by him to grow out into the ventricle and to pass backwards enclosed 
in a common sheath to meet with a similar bundle of nerve fibres 
growing forward from a group of large cells (“terminal canal cells”) 
actually situated within the terminal sinus. 
