44 



VERTEBRATE PHOTORECEPTORS 



6 



dyes; the inner segment shows preference for basic dyes. 

 The staining reaction of the outer segment varies with the 

 conditions of ilhimination. 



Rods possess an elUpsoid or lentiform body at the distal 

 eid of the inner segment. In fishes and anuran amphibians 



it is a single plano-convex 

 element, truncated on its 

 distal surface (v. Arey, 

 1928, plate 1). Most rods 

 ri P n lack a paraboloid. This 



^ - structure, however, is pres- 



ent in the rods of sala- 

 manders, turtles, Spheno- 

 don (Walls, 1934a), and 

 in the geckos (Detwiler, 

 1923c) where it is a very 

 prominent barrel-shaped 

 structure and much larger 

 than the ellipsoid (Figure 

 32, D). Arey (op. cit.), in 

 discussing the rods says 

 (p. 893), ''Lentiform bodies 

 may well play a dioptric 

 role. . . . Many of their 

 properties suggest this, and 

 the ellipsoid-paraboloid 

 combination is most interesting from such a viewpoint. Its 

 components have geometrical forms like those of the com- 

 ponent lenses in an achromatic objective, and both are more 

 refractive than the surrounding medium. ..." 



Apparently the single or compound lentiform apparatus 

 may serve to concentrate light upon the outer segments and 

 perhaps cause it to diverge as it passes the focal plane. The 

 colorless oil droplets in the cones may act in a similar way. 

 Rods are present in all vertebrates except the diurnal 

 lizards. In diurnal turtles they are generally regarded as 

 being absent, although Walls (1934a) claims that rods are 



A 



Fig. 32. Rods from various verte- 

 brates. A, catfish; B, frog; C, alligator; 

 D, nocturnal lizard (Gecko); E, chicken; 

 F, monkey; n, nucleus; m, myoid; 

 p, paraboloid; e, ellipsoid; o.s., outer 

 segment. (Figures are not drawn to 

 same scale.) 



