STRUCTURE AND DEVELOPMENT OF REISSNER’s FIBRE. 97 
Reissner’s fibre arises at the forward extremity of the 
mid-brain in a very large number of exceedingly fine fibrillse 
from the cells of the sub-commissui’al organ surrounding the 
sub-commissural canal. These join together to form fine 
threads, which, in one specimen, are definitely seen as a pair 
of larger factors. These are situated nearly centrally in the 
sub-commissnral canal, and become stouter as they pass back- 
wards from their brush-like origin. At a point near the 
middle of the length of the sub-commissual canal the lesser 
fibres join np into the very definite fibre of Reissner, the 
diameter of which is seen to increase markedly as it is 
followed backwards. 
In the one series cut transversely the fibre could not be 
followed. In another series (cut sagittally) it has sprung for- 
wards from the caualis centralis, to lie, as a thickened rod, 
in the dorso-posterior portion of the sinus mesocoelicus. 
In both of the remaining specimens it has preserved its 
normal position, and may be observed as a tautly stretched 
thread (fig. 22, r.f.) passing from the snb-coinmissural canal 
through the upper chamber of the sinus mesocoelicus and 
so into the isthmic canal. From this it emerges into the fourth 
ventricle and passes backwards along the lower of the two 
divisions of the canalis centralis of the spinal cord (6gs. 
31, 32, r.f.). In the section photographed for fig. 21 a con- 
siderable length of Reissner’s fibre occurs in the isthmic 
canal, but the magnification is too small to render the fibre 
visible. 
The sub-commissural organ, owing to the complete 
fusion of its two halves ventrally as well as dorsally, has 
assumed the shape of a test-tube with its sealed end forward 
(the sub-commissural canal, figs. 21-23, and Text-fig. 7, s.c.c.). 
It slopes backwards and downwards beneath the posterior 
commissure, which is very ill defined and which appears to 
extend through nearly half of the length of the mid-brain. 
Behind the posterior commissure the upper half of the sub- 
commissural organ bends dorsally into the optocoel, where it 
passes gradually into a more ordinary columnar epithelium. 
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