286 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Migration in the Proximal Direction (in the Light). 
Time. Ohr. | 4 hr. | hr. | $ hr. 1 hr. | 14 hr. 14 hr.| 13 hr.| 2 hr. 
Length of distal process . | 30 | 85 | 55 | 80 | 100 | 115 | 125 | 130 | 180 
Length of cellbody . . . | 70 | 70 | 65 | 60 50| 40| 365 30, 30 
Length of proximal process | 60 | 55 | 40 | 20 | 10 5 0 0 0 
The changes induced in the distal retinular cells by the light are 
completed, then, in a period between an hour and a halfand 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. 
Szezawinska (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 (791, 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 (01, p. 455) believes that in Palamo- 
netes the changes are 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 migration 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 u in two hours is exceptionally slow for the action 
