336 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
vehicle for conveying visual red to the rods. According to the obser- 
vations and deductions of these investigations the pigment would 
therefore assume a secondary réle, while visual purple, or some other 
substance in the photoreceptive elements, would become the chief 
factor in transforming the light-energy into the appropriate stimuli. 
The theory now most generally accepted (by Stefanowska, ’90, 
Szezawinska ’90, Exner ’91, Parker ’95 and ’99, Hesse :02, Garten :07, 
and Doniselli :07) is that the pigment is a protective mechanism for 
regulating the amount of light entering the receptive organ, and for 
rendering better definition to the image by preventing irradiation. 
This, however, still leaves open the question as to how the pigment 
migrates. What are the mechanics of the phenomenon? This prob- 
lem may be resolved into two questions:— first, whether or not the 
migration is under the control of the central nervous system; 
secondly, whether the migration is due to an intra- or to an 
extracellular activating force. 
As to the first question, Engelmann (’84), Fick (’90), Kiesel (’94), 
Herzog (:05) together with Nahmacher, Lodato, and Pirrone (cited 
by Herzog) have obtained affirmative evidence, whereas the investi- 
gations of Hamburger (’88), Fick (’91), Parker (’97), and v. Frisch 
(:08), on the contrary, have indicated that the migration is independ- 
ent of the central nervous system. Since some of the evidence is 
convincing for both views, it is possible that the two views are recon- 
cilable on the assumption that the migration of the pigment is induced 
primarily by the direct stimulus of light, while the central nervous 
system exerts a secondary influence upon it, perhaps of slight inhibi- 
tion or of occasional stimulation. 
The evidence both for an intra- and an extra-cellular cause of the 
migration I shall consider with respect, first, to the direct response of 
the pigment, and secondly, to the indirect response through a reflex. 
1. DrREcT RESPONSE OF THE PIGMENT. (a) Hvidence for an intra- 
cellular activating force—— Kiihne (’78) observed that after injecting 
pigment into the blood of a salamander those white corpuscles which 
had ingested a few particles of pigment were more kinetic under the 
influence of light than the others, which had not. Herzog (:05) 
believed that the pigment absorbed heat and emitted it in the form 
of either kinetic or chemical energy, which might stimulate the 
receptors or else might be but an indifferent form into which the 
radiant energy had been diverted. This view, according to which 
the migration would be proportional to the heat-energy absorbed by 
the pigment, is contradicted by the results obtained with colored 
