298 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



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, as 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 Palse- 

 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 Palsemon, 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, together with 

 the facts set down in the present paper, confirm me in the belief that 

 Eosenstadt's explanation of the migration of the pigment is erroneous, 

 and that the one presented in the foregoing account is correct. 



