STRUCTURE AND DEVELOPMENT OF RETSSNEr’s FIBRE. 71 
fibre, though not easily followed continuously through 
the brain-ventricles, may nevertheless be followed under 
the cerebellum, and becomes easy to trace in the can alls 
centralis of the spinal cord, where it has a diameter of 
almost 1 /n. 
In an aminocoete 65 mm. in length, the only important 
feature to note is the development of the isthmic canal upon 
tlie ventral surface of the I’hombo-mesencephalic fold. This 
canal in this specimen shows distinct traces of a paired 
character (fig. 44, i. c.). 
The right and left halves of the sub-commissural organ 
have become still more widely separated, and each is now 
slightly hollowed out to form a groove, the ventro-lateral 
borders of which are marked off very distinctly from the 
general ventricular epithelium. 
Owing pi’obably to the larger size of this specimen, pene- 
tration by the fixing fluid seems to have been less rapid, 
and the fibre is not quite so well preserved. It has, in the 
canalis centralis, a diameter of barely 1//. 
Another specimen, 95 mtn. long, cat, like the last, trans- 
versely, shows some advance upon the condition just 
described. The snb-commissural organ consists now of a 
pair of very definite grooves, each somewhat crescentic in 
transverse section (fig. 43, s. c. o.). As in all the other speci- 
mens examined, the sab-commissural organ begins in front 
on the left side, in the cleft between the habenular ganglion 
and the optic thalamus, and on the right side, somewhat 
further back, behind the habenular gaTiglion. The two 
grooves lie widely apart, have a nearly mesial presentation, 
and both extend very slightly behind the posterior commissure. 
This latter structure has enlarged considerabl}’, and 
owing to the marked growth of the habenular ganglia, its 
upper surface now lies at a somewhat lower level than the 
dor.sal surface of those ganglia. Further, from behind, the 
tela choroidea II has continued to grow forward and 
upward. The posterior commissure, therefore, being left 
behind by the more rapid gi-owth of the adjacent structures. 
