BETIUTA OF THE BLOWFLY. 
409 
Berger and Yiallanes trace the optic-nerve fibres through a 
series of small round cells, very conspicuous in the outer half of 
my retina, Hickson’s periopticon. Hickson regards these cells 
as of quite secondary import. They clearly belong to the sup- 
porting tissue and are external to the sheaths of tbe retinal 
elements, which are continuous with the perineurium of the optic 
nerve. 
Dr. Hickson and Dr. Grenadier suppose the sheathing cells of 
the great rods, retinulse of Grenadier, to be the nerve-terminals; 
and more recently Platten pretends that the optic nerve ter- 
minates in the crystalline cone. There is therefore no unanimity 
amongst previous writers, especially in matters of detail ; as it is 
impossible that they can all be right, it is quite possible, as I 
assert, that they are all wrong. 
Dr. Hickson’s neurospongium, or terminal anastomosis, which 
is inadmissible on physiological grounds, is no nerve-plexus 
at all, but the tracheal plexus and the sustentacular framework 
of my retina. 
It is exceedingly difficult to prepare sections which show the 
true retinal end-organs. This difficulty arises from the fact that 
the chloroform and alcohol used in the process of imbedding 
dissolve the fatty matters from the nerves, and the external 
extremities of my retinal rods are completely dissolved or dis- 
integrated by the action of aqueous media. 
I have, however, on many occasions succeeded in obtaining 
sections in which both the inner and outer extremities of the 
retinal rods, as well as the nerves, remain more or less unaltered. 
Another difficulty arises from the extreme transparency of these 
structures in very thin sections, and from the fact that they 
cannot be stained by any of the stains used in such researches ; 
the outer ends of the rods are not affected by strong solutions of 
aniline dyes, except vesuvin # . 
In thicker sections the numerous round cells between the 
retinal nerve-end organs, which are not connected with nerves, 
but with the sustentacular framework, entirely conceal the outer 
ends of the rods. 
There are two methods which give good results ; in both the 
tissues must be fixed either with osmic acid and absolute alcohol 
* The best demonstration of these organs is obtained by staining with a 
solution of vesuvin in aniline water. The solution must be quite freshly made, 
and unfortunately such preparations fade rapidly when mounted in balsam. 
