day: pigment-migration in eye of crayfish. 337 



lights of equal intensity. The phenomenon of an electric current 

 induced in the eye of a frog by light, studied by Gotch (:03 and :04), 

 might offer a basis for the interpretation of the migration. If there 

 were a difference in potential produced in the pigment-cell it would 

 not be one comparable, however, to that set up in the radio-micrometer 

 proportional to the radiant energy absorbed, but rather to that 

 created by some chemical change in the cytoplasm. Von Frisch 

 (:08), however, obtained no pigment migration in the eye of a shrimp 

 upon applying electrodes to it. 



(b) Evidence for an extracellular or chemotropic force} In many 

 insects and crustaceans there is a migration of distal pigment inward 

 and of proximal pigment outward, but in both cases toward the rhab- 

 dome. In the vertebrate eye the migration is outward; the pigment 

 surrounds the outer segments of the rods and cones, and stops pretty 

 definitely in the vicinity of the ellipsoids of the rods. The ellipsoids 

 of both elements stain less deeply with acid eosin in the light-eye than 

 in the dark-eye, indicating that a chemical change from an alkaline to 

 an acid condition has been produced by the light. As Birnbacher 

 ('94) pointed out, however, this condition in the ellipsoids might be 

 but the end of a series of changes which have gone on in other parts 

 of the rods and cones. Thus substances, chromatically visible or 

 otherwise, in the outer segments might also be concerned in the 

 chemotropism. 



2. Indirect response through a reflex. Since Fick ('90) and 

 Kiesel ('94) observed in the frog and moth respectively a migration in 

 the dark, it would seem that the optic nerve has a motor function. 

 Although I do not know whether there are any nerves connecting 

 with the pigment-epithelial cells in the vertebrate eye through which 

 the motor impulse might be dispatched, in the arthopod, according 

 to Parker ('95) and Hesse (:01), neurofibrillae pass up through the 

 substance of the proximal retinular cells and into the rhabdome. 

 Whether the distal retinular cells are likewise innervated, I do not know. 

 Such nerve-connections would mean an intracellular stimulus to mi- 

 gration in the dark as opposed to an extracellular chemotropic stimulus 

 to migration in the light. Even if the migration in the light were 

 evoked through a reflex, there might be a causal relation between 

 the chemical change evident in the retina and the initiation of this 



' By chemotropic force I mean an attraction-force (whether due to a change in 

 electric potential, osmotic pressure, aflinity of one chemical substance for another, 

 or similar process) occasioned by the dissociation of some chemical substance in the 

 receptive organ under the influence of light. 



