296 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
length of the cell, including the retinal nerve fibre. There are slight 
concentrations of pigment at the distal end of the cell and around the 
rhabdome. In the dark the pigment is limited to the retinal nerve 
fibres. 
3. The change from the dark condition to the light one is accom- 
plished in from 30 to 45 minutes. The reverse change requires 45 to 
60 minutes. 
4. These changes are probably due to internal protoplasmie move- 
ments, by which the pigment granules in the cells are moved in one 
or other direction. 
5. The accesssory pigment cells contain a yellowish white pigment. 
In the light this is massed partly in the base of the retina, and partly 
near the distal surface of the first optic ganglion. The two pigment 
masses are connected by pigmented strands. Jn the dark the pigment is 
almost entirely in the base of tho retina. 
6. The change from the dark condition to the light one is accom- 
plished in from 45 to 60 minutes; the reverse change, in from 105 to 
120 minutes. 
7. These changes are probably produced by amoboid movements of 
the cells. 
8. The distal retinular cells contain black pigment granules. Jn the 
light they are contracted, and occupy a proximal position in the retina 
f the ommatidium near the outer ends of the 
In the dark they are expanded (flattened), 
n in the retina, surrounding more or less 
surrounding the axis o 
proximal retinular cells. 
and occupy a distal positio 
completely the sides of the cone. 
9, The change, from the dark condition to the light one is accom- 
plished in from 90 to 105 minutes; the reverse change requires from 
105 to 120 minutes. 
10. These changes are produced in part by an amoeboid movement of 
the cell, and probably in part by a muscle-like contraction of its axial 
portion. 
11. Each set of photomechanical changes carried out in the light is 
completed in less time than the corresponding set of reverse changes 
carried out in the dark. 
12. The photomechanical condition of the retina in one eye has no 
effect upon that in the othér eye; i. e. the retinas are not sympathetic. 
