STEUCTURK AND DEVELOPMENT OF EEISSNEr’s FIBRE. 53 
from the istlimic canal and extends perfectly freely through 
the fourth veutricle, from which it passes into the canalis 
centralis to end in the sinus terminalis. In no part of 
its course does it penetrate the braiu-tissue, and the isthmic 
canalj in all of the seven series examined^ remains widely open 
below. In two series of sections the fibre had evidently broken 
from its attachment to the sub-commissural organ during 
the dissection made to expose the brain, and has sprung back- 
ward into the central canal of the spinal cord. In both 
cases, however, the process of fixation must have been well 
advanced, especially in the relatively less bulky spinal cord, 
and the fibre has consequently retracted only from the brain 
into the anterior region of the cord. In one case the recoil 
has merely resulted in a shortening up of the anterior part 
of the fibre and a considerable increase in its thickness in 
that region. In the second case the released anterior end 
of the fibre has been twisted into a spirally coiled mass of 
swollen fibre which lies at the point where the fourth ventricle 
])asses into the canalis centralis. It is associated with 
a considerable quantity of dislodged ependymal cells, and on 
e.^amluation it was found that the sub-commissural organ 
is somewhat incomplete, as though patches of cells had been 
dragged away when the fibi-e tore itself fi-ee. 
Throughout the extent of the canalis centralis the 
fibre is attached, at frequent intervals, to the- ependymal 
e[)ithelium by cilia, which appear to have fused with the 
fibre, and which probably are, as I have suggested above, 
actually constituent parts of Reissuer’s fibre. 
I have been able to trace Reissner’s fibre with absolute 
certainty to its actual end in the sinus terminalis in this 
species in but two specimens. This is due to the fact that 
the fibre can only be certainly followed in sagittal sections, 
the preparation of which, in this region, presents some diffi- 
culty. Tlie spiual cord extends as an exceedingly delicate, 
tapering filum terminale to the extremity of the tail. It 
is supported ventrally by the notochord, beyond the posterior 
end of which, however, it projects slightly, there becoming 
