DAY: PIGMENT-MIGRATION IN EYE OF CRAYFISH. 335 
covery by Birnbacher ('94) that the ellipsoids stain deeply in a dark- 
eye, but poorly in a light-eye with an acid stain; and the observations 
by Hess (:07 and :10) of a shortening of the range of perception in the 
blue end of the spectrum among birds and reptiles due to the colored 
oil-drops in the ellipsoids of the cones, all go to indicate that the outer 
segments of the rods and cones together with the ellipsoids are proba- 
bly the receptive organs. 
Since the pigment migrates outward to surround the rhabdomes 
in arthropods and the outer segments of the rods and cones in verte- 
brates, it evidently plays some role in the vision of the animal. 
Early investigators were tempted to attribute to it a primary 
function, that of transforming the light-energy into a form appropriate 
for stimulating the nerve-endings on the receptive organs. Kiihne 
(78) suggested that this might be by mechanical friction of the pig- 
ment-needles on the rods and cones, or perhaps by the end-products 
of chemical decomposition of the pigment under the influence of light. 
No evidence has been found for the mechanical view. As to the 
second view, support might be found in the following facts: — Kiihne 
(78) succeeded in bleaching pigment extracted from the eye and 
exposed to sunlight for several weeks; Stefanowska (’90) observed 
that in some insects (e. g., Eristalis tenax) the pigment was resolved 
into oil-drops, while Chiarini (:04) found that in many animals it 
diminished in quantity under the influence of light; and Raehlmann 
(:07) beheld under the ultramicroscope an actual bleaching of granules 
in the processes of the pigment-cell. Kiihne, however, never obtained 
bleaching in the living eye, and that which Raehlmann observed was 
in granules other than of pigment. Furthermore, since we know that 
pigment is unnecessary for the perception of light or color in many 
invertebrates, according to the observations of Hesse (:01), Mast (:11) 
and others, and likewise in vertebrate albinos, further that the rate 
of migration is relatively slow, and that vision is good after the migra- 
tion has ceased, the theory ascribing to the pigment a primary role of 
chemically stimulating the visual cells seems hardly tenable. 
Another theory attributes to the pigment the quasi-nutritive 
function of replenishing the exhausted visual cells. Kiihne found 
that an excised eye of a frog with the pigment-epithelium intact 
regenerated the visual red, while a retina from which this epithelium 
had been removed did not. Chiarini (:06) believed that the migration 
was due to chemotropism and that the pigment thereby replenished 
the rods and cones, whose substances had been exhausted by the 
influence of light. Raehlmann (:07) thought that the pigment was a 
