283 



reverse movement, in the dark, required from forty-five minutes to 

 an hour. 



In the accessory cells and the proximal retinular cells, the migra- 

 tion of the pigment is not the result of a change in the position of these 

 cells, but is due, I believe, to protoplasmic movements within the cells 

 themselves redistributing the pigment. In the distal retinular cells, 

 which are entirely filled with pigment, the changes involve an altera- 

 tion in both the form and position of these cells. 



In an eye subjected to light, the distal retinular cells (Fig. 1, cl. 

 dst.) surround the axis of the ommatidium immediately in front of the 

 proximal retinular cells. In this condition the body of each cell has a 

 length of about 30 |jl, and from it a single process, containing more or 

 less pigment, reaches distally to the corneal hypodermis. In an eye 

 kept in the dark (Fig. 2), the distal retinular cells occupy a position 

 some 70 [x further distal and the body of each cell has a length of 

 about 70{JL. In this condition the cell possesses, in addition toits distal 

 process, a proximal one which reaches the distal ends of the proximal 

 retinular cells. The proximal movement and the contraction of these 

 cells under the influence of light, is completed in from one and a half 

 to one and three-quarters hours. The reverse movement and the ex- 

 pansion of these cells, both of which changes occur in the dark, require 

 from one and three-quarters to two hours. 



When in a given animal one eye is exposed to light at the same 

 time that the other is kept in the dark, the pigment of each eye ad- 

 justs itself to its appropriate condition, thus demonstrating the inde- 

 pendence of the two eyes in this respect. 



The results of pigment migrations can be observed in retinas of 

 optic stalks which have been cut from the animals. Hence the brain is 

 not essential to these changes. Similar conditions can likewise be ob- 

 served in retinas cut from the optic stalks. The essential mechanism 

 of these changes must, therefore, be located entirely in the retina, and 

 is not only independent of the brain, but also of the ganglia in the 

 optic stalk. 



The preceding statements require one qualification. In many of 

 the experiments on excised optic stalks and excised retinas, the 

 pigment-changes, though noticeable, were only partial. This is due, 

 I believe, not to the withdrawal of any influence from the central 

 nervous organs, but to certain unavoidable conditions in the experi- 

 ments — stoppage of the circulation, etc., accompanied with a gradual 

 death of the tissues, which overtakes the retina before the pigment 

 cells have completed their changes. That the central nervous in- 

 fluences are not at all necessary to the migration of the pigment, is 



18* 



