286 bulletin: museum of comparative zooloCxY. 



Migration in the Proximal Direction {in the Light). 



The changes induced in the distal retinular cells by the light are 

 completed, then, in a period between an hour and a half and an hour 

 and three quarters long. The changes that take place in the dark require 

 for their completion from an hour and three quarters to two hours. 



Rough estimates of the time necessary for the completion of these 

 changes in different arthropods have been made by various investigators. 

 Szczawinska ('91, p. 552) states that in Astacus the condition charac- 

 teristic for the dark is reached in six hours, that for the light in two 

 hours. Exner ('91, p. 70) states that in an insect, Lasiocampa, the 

 changes require about half an hour, and Kiesel ('94, p. 105) gives the 

 same time for Plusia. Herrick ('91, p. 455) believes that in Paleemo- 

 netes the changes ai-e accomplished in about twenty-five minutes, an 

 estimate that I should regard as rather too low\ 



Exner ('91, p. 70) has suggested that muscle fibres might be con- 

 cerned in the migration of the distal retinular cells, an idea that gains 

 some support from the fact that in the eyes of some insects structures 

 like muscle fibres have been seen and described. In the crustacean 

 retina, however, Exner was unable to find anything like muscles. At 

 first sight it might seem probable that what I have described as the 

 proximal and distal processes of the distal retinular cells might be mus- 

 cular in nature. But the facts that the proximal process disappears 

 entirely during the proximal migration of the cell, and that the distal 

 one seems never to be firmly attached near the periphery of the retina, 

 are opposed to this view. Moreover, in the distal process, which, on the 

 whole, is the more muscle-like of the two, I have been unable to discover 

 any evidence of transverse enlargement in the shortened condition, such 

 as a contracted muscle exhibits. The cell in its distal migi-ation seems to 

 move over the fibre rather than to be drawn onward by a contraction of 

 the fibre. Further evidence against the muscular nature of the motor 

 mechanism of these cells is to be found in the rate at which the move- 

 ment takes place ; 50 /x in two hours is exceptionally slow for the action 



