PARKER: RETINAL PIGMENT CELLS OF PAL^MONETES. 283 



the retina, and the other near tlie distal surface of the first optic 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 heing only 

 a few small strands proximal to the basement 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 photomechanical 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 

 amoeba. AVhen the retina is placed in the light, the cells with their 

 contained pigment creep slowly backward through the apertures in the 

 hasement 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 amoeba 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 ; hut I have been 

 unable to get conclusive evidence as to which it is. 



