PARKER: RETINAL PIGMENT CELLS OF PALZEMONETES. 281 
minutes, it was found throughout the bodies of the cells; and, at thirty 
minutes, well marked concentrations had appeared about the rhabdome 
and at the distal end of each cell. At forty-five minutes, these concen- 
trations were somewhat more pronounced, but after that time no further 
changes were observable. 
The revorse change, which takes place in the dark, is accomplished in 
the following manner. After the animal has been in the dark fifteen 
minutes, the concentrations of pigment about the rhabdomes and at 
the distal ends of the cells have almost disappeared, though the bodies 
of the cells still contain an almost uniform amount of pigment through- 
out their whole length, After thirty minutes, much more pigment is to 
be found proximal to the basement membrane than distal to it, and after 
forty-five minutes almost all the pigment is proximal in position. At the 
end of an hour, the condition characteristic of darkness is fully realized. 
The changes just recorded occur entirely within the limits of each 
proximal retinular cell. There is no reason for believing that the 
changes are the results of a process of pigment production in one part 
of the cell, and of pigment destruction in another. The observed facts, 
on the contrary, suggest that the pigment granules of one region in the 
cell are moved to another. The movement, however, is not accompanied 
by any noticeable change in the position or even the form of the contain- 
ing cell. The pigment granules seem to be carried up and down through 
the cell as though by a streaming of the cell protoplasm. A similar 
stability of form, accompanied with an internal movement of pigment, 
has been described by Ballowitz ('93, Taf. XXXVI. Fig. 12, and '93*, 
p. 629) in the pigment cells of the skin of fishes. 
Through the kindness of Professor F. H. Herrick, I have had the priv- 
ilege of examining an interesting series of eyes taken from specimens of 
Palwemonetes that had been kept living in a dark chamber thirty-eight 
days. The pigment in the proximal retinular cells of such animals 
showed the condition characteristic of the dark. In an animal that had 
been kept in the dark for this period and then exposed to light for four 
hours and three quarters, the pigment returned partially to the position 
characteristic of the light. The greater part of it remained proximal to 
the basement membrane, and from that which moved into the bodies of 
the cells no marked concentrations were formed, either about the rhab- 
domes or at the distal ends of the cells. Long confinement in the dark, 
then, seems to interfere somewhat with the mechanism by which the 
pigment of these cells is normally moved. 
The accessory pigment cells are located in the base of the retina, and 
