STRUCTURE AND DEVELOPMENT OF REISSNER’s FIBRE. 105 
speaks of the optocoel as the “cerebellar ventricle/’ and 
clearly regards the isthmic canal as part of the fourth 
ventricle, and claims to have found numerous blanches of 
the fibre issuing from the “ mid-brain ventricle ” (the sub- 
commissural canal of my descriptions) and the “cerebellar 
ventricle” to join a main fibre. This main fibre was found 
only as a much coiled thread in the “ fourth ventricle ” (the 
sinus mesocoelicus and isthmic canal of my descriptions). 
He speaks, moreover, of having examined the fibre “ in 
section and by dissection” (my spaced type), which is 
rather a remarkable statement, for in none of my specimens 
does the fibre exceed 2 /x in diameter, and it is visible only 
under quite considerable magnification. It is clear, however, 
from his description, that the fibre must have been broken or 
cut prior to the preservation of his material, and that a con- 
siderable length had retracted into the sinus mesocoelicus. 
Some loose loops, perhaps, may have been thrown into the 
optocoel and so account for the condition he describes. As 
already stated I found only in a single specimen (B. stouti) a 
very delicate branch coming from the optocoel and joining 
Reissner’s fibre as it emerges from the sub-commissural 
canal. 
The only real addition to our knowledge of Reissner’s fibre 
made by Ayers is contained in his statement that the ultimate 
fibrillae are derived from the cells of the ependymal epithe- 
lium. Unfortunately great importance cannot be attached 
to his account, for it is far from certain that much which he 
has interpreted as “ ventricular fibre ” is not really coagnlum. 
In Petromyzon, both in larva? and adults, he states that 
he was unable to find these fibres. In their place he found 
“a fine-meshed network of fibrils . . . which bears the 
same relations to the ependymal cells and in life practically 
fills the ventricular cavity.” In this case there can be no 
doubt, from his description, that he refers to the fibre-like 
coagulum which commonly occurs in the brain-ventricles, 
and lias altogether overlooked Reissner’s fibre. 
He suggests that the function of these fibrillae is to bring 
