STRUCTCTIIE AND DEVELOPMENT OF REISSNEU’S FFRRE. 73 
In the last few sections the spinal cord narrows into a filnin 
term inale (fig. 48), in which the canalis centralis 
gradually widens out into a space, almost square in transverse 
section, which is enclosed only by a columnar ependymal 
epithelium, and which must be the sinus terminalis 
(fig. 49, s.t.). 
This terminal chamber extends through six sections only 
(60 micra), and has a maximum diameter of 8//. It is abso- 
lutely free from contained cells of any kind in my youngest 
specimens. 
In many sections through the canalis centralis 
Reissner’s fibre can be made out as a very minute dark dot, 
and may be traced backwards to the sinus terminalis, 
where it is lost. 
From what has been stated abov'e of the size and character 
of the lumen of the central canal at this age, it will be 
obvious that it is scarcely practicable to trace the fibre in 
sagittal sections through tlie tail region, for even where it 
happened that the plane of the sections was exactly sagittal, 
the canal was invariably contained within the thickness of a 
single section. Only in the sinus terminalis, therefore, 
coidd the lumen be satisfactorily examined, and in this, in 
both of the 13 mm. specimens and also in the 14 5 mm. 
specimen, Reissner’s fibre may be made out as an extreniely 
delicate thread, which in the longer of these three specimens 
ends in a swollen knob. This knob appears to be a mass of 
coagulurn (fig. 50, /.p.), and in it inay be distinguished the 
remains of nuclei. From the condition, however, in one of 
the 13 mm. specimens, it would appear that this knob ma}’- 
really represent the terminal plug, which would then 
apparently have a cellular origin. 
In both cases this terminal plug, if it be such, has become 
detached, and in the smaller specimen it has also apparently 
been displaced slightly in the preparation of the sections. In 
both also the sinus terminalis opens posteriorly by a small 
gap in the ependymal epithelium, at a point external to which 
lies a group of imlifferent cells (Fig. 50, m. which I’epresent, 
