24 
G. H. Parker 
beyond doubt (PI. 1 Fig. 6, cl.dst]. These pigmented processes, 
as might be expected, are not new structures, but are the trans- 
parent processes of the distal retinular cells partly filled with pig- 
ment from the body of the cell. The largest migration that the 
pigments made, was over a distance about equal to the length of 
the cone. Exner (91, pag. 73) states that, after the proximal migra- 
tion has taken place, only a few traces of pigment are left between 
the cones; and Szczawinska (91, pag. 552) says that the change 
renders the pigmented cylinder of the cones in part imperfect. 
Although the subtraction of pigment granules from between the cones 
may render that region somewhat less impervious to light, certainly 
more pigment remains behind than passes into the processes, and 
I never observed such a deficiency in what was left as Szczawinska 
(91, PI. 17 Fig. 2) figures. In this case, then, the change from dark- 
ness to light induces the migration of a small part of the pigment 
from the body of the cell into its proximal process. 
The opposite extreme to the case just described was as follows: 
the pigment of the distai retinular cells, when prepared in darkness, 
occupied not only the body of these cells, but also the distai portion 
of their proximal processes , so that their appearance was almost 
identical with that produced by the action of light in the former 
instance. The light, in this case, caused the pigment in the processes 
to migrate stili farther tili it nearly touched the proximal retinular 
cells. In both cases, then, the light produced the same effect, a 
proximal migration of the pigment; in the first instance, this 
began in the body of the celi, but, in the second, it began in the process 
itself. Since in these experiments the action of light and of darkness 
was tried one on the right and the other on the left eye of the 
same crayfish, and since in ali preparations the pigment of the 
proximal retinular cells reacted normally and fully, I believe that I 
am justified in interpreting the conditions in these two extreme cases 
as due to individuai variations and not caused by pathological changes. 
This conclusion is supported by the fact that, in the four pairs of 
eyes that I bave cut, the changes in the pigment of the distai reti- 
nular cells show considerable Variation as contrasted with the uni- 
formity of the changes in the proximal cells. 
Although the pigment in the distai retinular cells migrates in 
the direction that Exner has stated, the fact that it has the form of 
a narrow process instead of a cylinder might at first sight seem to 
offer a serious objection to bis explanation of these changes and 
