PARKER: RETINAL PIGMENT CELLS OF PAL^MONETES. 279 



none proved so satisfactory as water at about 80° C. The periods of 

 exposure to dark in the first set of experiments were as follows : 1 min., 

 5 min., 10 min., 15 min., 30 min., 45 min., 60 min., and then at intervals 

 of an hour up to 8 hours. It was found subsequently that the experi- 

 ments need not have extended over a maximum period of moi-e than 

 two hours, and that intervals of about fifteen minutes were all that 

 were needed to observe the steps of the change. From each lot of ani- 

 mals prepai-ed in this way, the optic stalks were cut into sections for 

 examination under the microscope. In a similar way, the eyes of ani- 

 mals that had been kept some four hours in the dark were exposed to 

 the light for given intervals, killed, cut, and examined. In cases where 

 it was necessary to make very accurate comparisons, the eyes of the 

 same animal were used for the two conditions ; thus, after keeping 

 the animal a given time in the light, one optic stalk was removed, and tlie 

 animal kept in the dark. At the expiration of the second interval, the 

 second optic stalk was removed and prepared. To guard against indi- 

 vidual variations, in every experiment the eyes of at least three animals 

 were examined. 



The only general changes shown by retinas subjected to light or dark 

 were changes in the arrangement of the pigment. In other respects 

 they were not noticeably altered. Thus, no change in thickness was 

 observable; in one case, a left retina that had been kept in the dark 

 measured in its middle region from the corneal cuticula to the basement 

 membrane 263 ^, while the right retina from the same animal exposed 

 to light measured 270 fi. In a second case, a dark left retina measured 

 240 fi, the light right one measuring 233 fi. The cones likewise showed 

 no significant differences. By analogy with the perceptive elements in 

 the vertebrate eye, one might have expected the rhabdouies, the termi- 

 nal nervous organs of the crustacean eye, to shorten in the light and 

 lengthen in the dark. I was unable to obtain evidence of such a change 

 in Pala^monetes, and yet the conditions for the exact measurement of 

 the rhabdomes are so unfavorable in this animal that I am by no means 

 certain that these changes may not occur. If, however, they do take 

 place, they must be relatively small. The observable changes induced 

 in the retina by the absence or presence of light affect the three kinds 

 of pigment cells, — the proximal retinular cells, the accessory cells, and 

 the distal retinular cells. These will be considered in the order given. 



The pigment in the proximal retinular cells forms at the base of the 

 retina a band, called by Exner ('91, p. 62) the retinal pigment. The 

 photomechanical changes that this pigment undergoes have already been 



