RETINAL PIGMENT OF PLANORBIS 



371 



Measurements shovoing the relative distribution of the retinal pigment of Planorbis 

 in darkness and in light at room temperature. The vahies are mean values ex- 

 pressed in micra, and indicate: (1) the thickness of the main pigment mass {zonal 

 measurement) ; (2) the zonal measurement plus the length of the pigmented processes 

 {process measurement); (3) the length of the pigment processes obtained by sub- 

 tracting {1) from {2). The percentage change in each mean measurement in 

 darkness as compared with that in light is also computed 



Since the distal margin of the pigment zone — the boundary 

 between central zone (fig. C, rtn. i.) and iniddle zone {rtn. m.) — 

 is fixed in relation to each pigment cell, these values show quite 

 decisively that in the light (fig. 2) the pigment migrates distally 

 in the cell, that is, toward the source of illumination, whereas 

 in the dark (fig. 5) the reverse is true. An appreciable thinning 

 out of the pigment u\ darkness due to its bemg spread over a 

 greater area, is not demonstrable. It may be that the pigment 

 involved in migration is only that which lies most proximally 

 in the middle zone, or there may occur a general proximal move- 

 ment, that is, a readjustment of the relative quantitative amounts, 

 throughout the whole mass. AMiichever alternative is true, 

 it is certain that the edge of the pigment zone which forms the 

 boundary between the middle and central zones (fig. C) main- 

 tains a constant position in relation to the retinal cells. 



2. Effect on excised eyes. Since light exercises a photome- 

 chanical influence on the retinal pigment, the interesting query 

 arises as to whether this effect is direct, upon the cell itself, 



restricting the measurements to this most favorable region afforded the fairest 

 numerical representation of the pigment distribution. In these, as well as all 

 experiments to be described subsequently, individuals of Planorbis were selected 

 of a more or less uniform size — this precaution will, I think, serve to obviate 

 criticism, if not actual error. 



