PARKER: RETINAL PIGMENT CELLS OF PALJEMONETES. 283 
the retina, and the other near the distal surface of the first optie ganglion. 
These two concentrations are connected by irregular bands of pigment. 
In the dark (Fig. 2) almost all the accessory pigment is in the base of 
the retina, the concentration near the ganglion as well as the interme- 
diate pigmented bands being represented by only a few small pigmented 
patches. 
The change from the condition produced by the light to that produced 
in the dark is indicated in the following steps. After the animal has 
been about thirty minutes in the dark, the concentration of pigment for- 
merly near the optic ganglion is appreciably nearer the retina, After 
forty-five minutes, this concentration as such has disappeared, and that 
in the retina has considerably increased. Finally, after two hours, almost 
all the accessory pigment lies in the base of the retina, there being only 
a few small strands proximal to the baserhent membrane. 
In the reverse change under the influence of light, the intermediate 
pigment strands show a perceptible thickening between ten and fifteen 
minutes after the eye has been placed in the light, and the full concen- 
tration at the level of the ganglion is completed within the period extend- 
ing from forty-five minutes to an hour after that event. 
I have never been able to discover any outlines to the accessory pig- 
ment cells except those indicated by the pigment mass itself. Judging 
from these, the photomechanieal changes in the accessory cells involve so 
radical an alteration in the forms of the cells that the latter may be said 
to have assumed a different position. In this respect, then, the pigment 
changes in these cells involve much more active movements than in the 
case of the proximal retinular cells, and possess something of a locomotor 
character. So far as I have observed them, they may be compared with 
perfect propriety to the more or less circumscribed movements of an 
amoba. When the retina is placed in the light, the cells with their 
contained pigment creep slowly backward through the apertures in the 
basement membrane toward the optic ganglion. When the retina is in 
the dark, they reverse this movement and creep out into the base of the 
retina. The one particular in which this movement differs from that of 
an amooba is that of its limitations in direction. Thus the cells always 
creep either outward or inward. Moreover, in darkness they do not 
creep indefinitely outward, but after about two hours reach a maximum 
limit ; the same is true of their inward course. These limitations may 
be due either to the structure of the regions into which the cells creep, 
or to the intrinsic qualities of the cells themselves; but I have been 
unable to get conclusive evidence as to which it is. 
