146 



ADAPTATIONS TO A RHYTHMIC ACTIVITY 



Pigment Migration — It will be recalled that the cells of the retinal 

 pigment epithelium often bear groups of long processes which interdigi- 

 tate with the visual cells (Fig. 20d and e, p. 44) and that in the latter 

 a portion of the inner segment between nucleus and ellipsoid is often 

 contractile and then bears the name of myoid (Figs. 22, 23, 24; pp. 54, 

 55, 59). It is the retinal pigment (fuscin) in the pigment-cell processes, 

 and the rod and cone myoids which are chiefly concerned in the photo- 

 mechanical changes of the retina. These changes are most conspicuous 

 in duplex retinae and are concerned with both light- and dark-adaptation 

 of the retina. 







Fig. 62 — Photomechanical changes in the retina of a fish, Phoxinus lavis. 

 From Kiihn, after von Frisch. 



a, visual-cell layer and pigment epithelium in light-adaptation, b, dark-adaptation. 

 e- pigment epithelium; r- rods; c- cones; /- limitans; n- nuclei of visual cells. 



When an animal equipped with photomechanical changes emerges 

 into bright light, a large portion of the retinal pigment — that which is 

 in the form of rodlets or short needles rather than tiny spherules — starts 

 to flow slowly down into the pigment-cell processes. These may be num- 

 erous and so slender that the granules pass into them in single file, or 

 they may be fewer and much more bulky. In as little time as half an hour 

 (though usually more slowly) the pigment will be found to be largely 

 scattered along the length of the processes and may reach nearly to the 

 external limiting membrane, being piled up into a dense mass at this 

 limit of its excursion. It thus forms a system of cylindrical sheaths sur- 



