THE FUNCTION OF REISSNER’S FIBER 143 
thelium, or Reissner’s fiber lying against it or beyond it. Usu- 
ally, however, the relaxed fiber does not lie, everywhere, in a 
perfectly straight line and, if one is actually dealing with the 
fiber, a careful tracing of the central canal will almost always 
show the fiber, sooner or later, turning centrally away from the 
wall of the central canal (fig. 27) and standing for a longer or 
shorter stretch as a distinct and free central thread. 
There would be less difficulty, perhaps, in certainly recogniz- 
ing the fiber if it invariably maintained its normal thickness (2 
to 3 w in the rays) or swelled, as it may do after being cut, to as 
much as 8 uw or 10 w in diameter. Not altogether infrequently, 
however, the fiber appears extremely fine, of a thickness which I 
estimate to be less than 0.3 u. Of such a diameter is the fiber in 
early development in larval cyclostomes and amphibians, and I 
can only conjecture that the occasional occurrence in these small 
rays of this delicate fiber is an indication that there has taken 
place a retraction of the fiber so extensive that repair has taken 
on the character of a completely new growth which is at first 
much thinner than in the adult state. 
In yet other cases the fiber may be wanting in the region ex- 
amined but there may be found, lying centrally, a shadowy 
structure which seems to be a hollow cylinder (fig. 18) whose 
diameter is considerably greater than that of a much swollen 
fiber. Were it not that a swollen and displaced length of fiber 
often lies nearby, I should have been disposed to regard this 
structure as the product of the disintegration of the fiber. Pos- 
sibly it represents a film of coagulated cerebro-spinal fluid which 
has formed around a swollen and gradually withdrawing fiber. 
The tissues were stained (in bulk) with borax carmine. 
Double staining was soon. abandoned as it was found that parts 
of thick sections were at times imperfectly fastened to the slide 
and were liable to be lost in the staining or decolorizing fluids 
and it was most important that no parts of Reissner’s fiber 
should be lost in this way. In the few cases in which double 
staining was resorted to, the second stian was invariably picro- 
indigo-carmine. 
