pakker: eetinal pigment cells of pal^monetes. 



285 



a whole. That these two elements are distinct can be seen from the fact 

 that in certain insects they are related to each other in a way just the 

 reverse of that which occurs in Paltemonetes ; thus, in Lasiocampa, as 

 figured by Exner ('91, Taf. IV. Figs. 28, 29), the distal cells are short 

 in their distal position, and long in their proximal one. 



The average length of the bodies of the distal retinular cells from the 

 left eye of a given animal prepared in light was about 30 n. The aver- 

 age length of the corresponding cells from the right eye of the same 

 animal prepai-ed in the dark was about 70 fi. In a series of preparations 

 taken from animals of approximately the same size as that just described, 

 the lengtheniug of the distal cells in the dark took place at a rate indi- 

 cated in the first of the following tables. 



In a second series the shortening of the cells under the influence of 

 light was shown to take place as indicated in the second table. 



The distal and the proximal migration of the cells are diiiicult to define, 

 because they are accompanied b}'^ the lengthening and the shortening of 

 the cells. Taking the nucleus as a fixed point in the cell, the maximum 

 distance of migration is about 50 j.i. In the migration from the proxi- 

 mal to the distal extreme, made in the dark, the cell ti'averses this 

 distance in about two hours. The migration in the reverse direction, 

 under the influence of light, is completed in about one hour and three 

 quarters. 



As the cells move outward, their distal processes shorten and their 

 proximal ones form and lengthen. As they move inward, their distal 

 ones elongate and their proximal ones shorten and finally disappear. 

 The rates of these changes, as well as of those given in the preceding 

 paragraphs, ai'e indicated in the following tables of summaries, in which 

 the varying lengths of the parts of the cells ai'e given for successive 

 periods. 



Migration in the Distal Direction (in the Dark). 



1 All measurements of length are expressed in mikra (thousandths of a milli- 

 meter). 



