day: pigment-migration in eye of crayfish. 335 



CO very by Birnbacher ('94) that the elHpsoids stain deeply in a dark- 

 eye, but poorly in a hght-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 



