142 GEORGE E. NICHOLLS 
ously obscure the lumen, and a structure so slight as is Reissner’s 
fiber normally can scarcely be distinguished with certainty, if 
viewed through the thickness of the epithelial wall of the filum 
terminale. In the swollen or spirally twisted condition the fiber 
becomes much more conspicuous, it is true, but even so there is 
still considerable difficulty in making out details. In order, 
therefore, to make reasonably sure of recognizing the fiber, the 
sections ought not to have a thickness greater than 20 nu. In 
such sections the fiber, if present, would be likely to appear as a 
well defined thread in an open canal and even if the sections 
should chance to include also an underlying or overlying layer of 
epithelium, this would almost certainly be quite thin. 
Accordingly, the attempt was made to cut the tails in sagittal 
sections of 20 u in thickness, the resulting series consisting gen- 
erally of comparatively thin sections alternating with others 
considerably thicker, the latter often permitting the presence 
(and extent) or the absence of the fiber to be ascertained, details 
being filled in from the thinner sections. 
As already observed, the almost invariable distortion ae ae 
material led, very generally, to parts of the filum terminale being 
cut very obliquely (fig. 26). Thus it happened sometimes, even 
where all the sections of a series were thick, that parts of the 
lumen of the central canal were exposed clearly to view. On the 
other hand, even where moderately thin sections had been ob- 
tained, there was occasionally some difficulty in deciding whether 
or no Reissner’s fiber was present. In experimental material in 
which the fiber has been broken, the relaxed fiber may frequently 
be found lying closely against the surface of the cells which line 
the central canal. This epithelium has a clear and highly re- 
fractive internal border which stains, with borax carmine, a 
delicate pink, precisely like a lightly stained Reissner’s fiber. A 
very slight alteration of the focus of the microscope produces, 
along the cut edge, the effect of a double line and gives rise to an 
appearance which may readily be mistaken for the fiber lying in 
juxtaposition, optically or actually, with the epithelial surface. 
It has been found impossible, in some cases, to be absolutely 
certain whether one is viewing the cut internal edge of this epi- 
