37 
then only when the after-treatment with iodine is not thorough. Such 
artifacts may have their shape more or less determined by the cavities 
in which they are formed, but then they are not easily misinterpreted. 
If the animal had been dead several hours before the cord and brain 
were placed in fixing fluid, the canalis centralis was found filled with 
coagulated granular matter, and the fibre was so far disintegrated as 
to be indistinguishable. | 
In investigating the course and occurrence of Reissner’s fibre I 
have made and studied series of sections of the central nervous system 
of more than 100 individuals representing upwards of 60 different 
species. In addition I have been able to draw upon a large collection 
of preparations of the central nervous system of Teleosts previously 
prepared. All the chief groups and principal sub-groups of vertebrates 
have been examined, and in no case where perfectly preserved material 
has been carefully studied have I failed to find Retssner’s fibre present. 
In most of the Ichthyopsida Reıssner’s fibre is relatively large 
averaging perhaps 3 « in diameter. In the group of Teleosts, where 
it has been most carefully studied, one finds the greatest variation in 
size, even in closely related species. In the tautog (Tautoga onitis), 
the fibre is conspicuous, having a diameter of 4 «, while in tne closely 
related cunner (Tautogolabrus adspersus) the fibre has a diameter of 
1 u or less. The diameter of the fibre in these cases seems to have 
a more direct relation to the size of the body than to the size of the 
brain, there being very little difference between the two species in the 
latter respect. For a given species, however, the size is remarkably 
constant. In upwards of one hundred series of Tautogolabrus studied 
I have yet failed to find Reissner’s fibre exceeding 1 w in diameter. 
The diameter of the fibre varies with the size (age?) of the individuals 
of a species, but not in the ratio of the body, being relatively much 
larger in the larval stages. The diameter of the fibre bears no direct 
relation to the diameter of the lumen of the canal, and in passing 
from the canal into the larger spaces of the 4th and 3d ventricles 
the fibre suffers no change in diameter. Throughout its course through 
the ventricles and canalis centralis the diameter of the fibre remains 
fairly constant, except near the extreme posterior end of the canal, 
where it diminishes in size, but not so rapidly as the canalis centralis itself. 
The following measurements of the diameter of the fibre and lumen 
of the canalis centralis show something of these relations. The me- 
asurements are in most cases given for three regions in the course 
of the fibre — the 3d ventricle, the anterior part of the cord, and 
the posterior part. 
