THE STRUCTURE OF VISUAL CELLS 43 



is of the opinion that they are incomplete division forms of a 

 single embryonic cell. 



The oil droplets are present in amphibians, reptiles, and 

 birds, but are lacking in fishes and mammals. When present 

 they lie at the distal end of the inner segment (Figure 30). 

 In reptiles and birds they are colored. Whereas all colors are 

 claimed to be represented, those towards the blue-violet end of 

 the spectrum are rare. They are most striking in birds where 

 Waelchli (1883) distinguished some eight varieties. Accord- 

 ing to Wald (19396), in the chicken they are red, golden, and 

 greenish yellow in color, forming a color filter arrangement 

 similar to that employed in many systems of color photog- 

 raphy. In birds also certain regional distribution of colors 

 may be made out, designated as the red field or the yellow 

 field. 



Rods. As with the cones, visual rods vary greatly in size 

 and general proportions in different animals. In the amphib- 

 ians, for example, the urodele rods are short, cylindrical 

 structures; the anuran rods are much longer. They are long 

 in fishes (light-adapted), birds, and man. The owl has no- 

 tably long rods (85 m) • In the human they are said to have a 

 length of 60 ix in the fundus, and 40 /x at the or a serrata. In. 

 many nocturnal forms they are actually filamentous (e.g. 

 rodents, lemurs). In Nyctipithecus (owl monkey) which 

 lacks cones, the rods are also very slender and closely packed 

 (Figure 54). However, in the bat the rods are quite short 

 (26 m), and much less filamentous (Detwiler, 1924, Figure 1). 

 Rods, like cones, possess an inner and an outer segment, 

 between which there exists in many forms a refractive disc 

 or intermediate plate. The inner segment consists of a myoid 

 and an ellipsoid. The myoid of teleost fishes, anuran am- 

 phibians, and birds is contractile, shortening in darkness 

 and elongating in the light. Measurements of the rods of 

 Ameiurus have shown a length of 92-93 ^ in light and 33.8 m 

 in darkness. The difference between the two conditions is 

 shown in Figures 57 and 58. The outer segment, which is 

 strongly refractive, tends to stain specifically with acid 



