PARKER: RETINAL PIGMENT CELLS OF PALZEMONETES. 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 Palemonetes; 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 h. The aver- 
age length of the corresponding cells from the right eye of the same 
animal prepared in the dark was about 70 u. In a series of preparations 
taken from animals of approximately the same size as that just described, 
the lengthening 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 difficult to define, 
because they are accompanied by 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 fl. In the migration from the proxi- 
mal to the distal extreme, made in the dark, the cell traverses 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 wellas of those given in the preceding 
paragraphs, are indicated in the following tables of summaries, in which 
the varying lengths of the parts of the cells are given for successive 
periods. 
Migration in the Distal Direction (in the Dark). 
Time. Ohr. I hr. | hr. | hr. 1 hr. 14 hr. 13 hr. 14 hr.|2 hr. 
Length of distal process . | 1301| 125 | 110 | 95 | 85 | 65 | 56 40 | 30 
Length of cell body . . . 80 | 85 | 50 | 55 | 60 | 70 | 70 | 70 | 70 
Length of proximal process 0 0 0 10] 15 | 25 | 35 | 50 | 60 
1 All measurements of length are expressed in mikra (thousandths of a milli- 
meter). 
