296 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



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 

 rhabdonie. 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 cliange requires 45 to 

 GO minutes. 



4. These changes are probably due to internal protoplasmic 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. In the dark the pigment is 

 almost entirely in the base of the 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 amceboid movements of 

 the cells. 



8. The distal retinular cells contain black pigment gi-anules. In the 

 light they are contracted, and occupy a proximal position in the retina 

 surrounding the axis of the ommatidium near the outer ends of the 

 proximal retinular cells. In the dark they are expanded (flattened), 

 and occupy a distal position in the retina, surrounding more or less 

 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 other eye ; i. e. the retinas are not sympathetic. 



