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 chemotroyic 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, affinity 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. 
