134 GEORGE E. NICHOLLS 
terminale in the Ichthyopsida. Elsewhere completely enveloped 
by the brain and spinal cord, Reissner’s fiber is peculiarly 
accessible at the extremity of the tail, the more so that there is 
practically an absence of nervous tissue in the hinder part of 
the filum terminale. This structure is, indeed, little more than 
a simple tube of columnar epithelium. At the actual hinder 
end, Reissner’s fiber may be said to be protected only by the 
Text-fig. 3 A sagittal section through the extremity of the tail of Raia blanda 
(III—experiment 3) to show the position of the sinus terminalis. c.c., cen- 
tral canal of the spinal cord (and terminal filament); f.t., filum terminale; mn., 
meninges, forming the hinder wall of the sinus terminalis; nch., notochord; R-f., 
Reissner’s fiber; s.t., sinus terminalis; t.p., terminal plug; v.c., vertebral column. 
skin and the delicate meninges, between which there lies but a 
film of connective tissue (text-fig. 3). 
A cut made in the vicinity of the end of the terminal filament 
would break the continuity of the fiber, therefore, but would be 
quite unlikely to produce physiological results such as to mask or 
interfere with the reactions resulting from the disorganization 
of the mechanism of which Reissner’s fiber forms a part. 
My experiments, then, were intended primarily as an attempt 
to determine the function of Reissner’s fiber and its related 
