298 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
pigment migrates from one cell to another is, so far as Rosenstadt’s ac- 
count goes, entirely unsupported by direct evidence, and seems to me 
an unwarranted assumption. The proximal movement of the pigment 
from the distal end of the retinula to the opposite side of the basement 
membrane is certainly accomplished within the limits of one set of cells, 
for, a8 I have shown in this paper, the pigment even when entirely 
proximal to the basement membrane lies in the thick retinal nerve 
fibres, which are merely processes from the proximal retinular cells. 
Although it cannot be stated with certainty that there is no exchange of 
pigment between the distal and the proximal retinular cells in Palæ- 
monetes, for in this crustacean in bright light these two kinds of cells 
are closely applied to each other, it is perfectly certain that in other 
decapods, as for instance Palemon, no such exchange is possible; for, 
as Exner (’91, Taf. V. Fig. 51) has shown, and I can confirm his obser- 
vations, the pigmented parts of the distal and the proximal retinular 
cells never touch, even under full light. "These reasons, togother with 
the facts set down in the present paper, confirm me in the belief that 
Rosenstadt’s explanation of the migration of the pigment is erroneous, 
and that the one presented in the foregoing account is correct. 
