PHOTOMECHANICAL RESPONSES 79 



owl and to the hen. The owl apparently possesses very 

 delicate cones, and the hen, although essentially a diurnal 

 form, possesses a goodly number of rods (Figure 45). 



In mammals, cone movements seem very doubtful. Van 

 Genderen Stort claimed positional changes in the cones of 

 the pig. Chiarini (1906) obtained negative results on the dog, 

 and Garten working with apes suggested possibly slight 

 changes in the region of the fovea. 



Cone contraction under the influence of light is greatest 

 in fishes; it occurs to a less extent in amphibia and in some 

 birds. Changes in the reptiUan cones are slight, but demon- 

 strable. In mammals the evidence is most doubtful. 



Rods. Angelucci (1884) was the first to observe positional 

 changes in the vertebrate rod. He claimed that the frog 

 myoid shortened after exposure to light. He applied the term 

 myoid to the contractile portion of the rod as Englemann had 

 done for the cone. Arcoleo (1890) also reported a shortening 

 of the rod myoid in light, but Lederer (1908) challenged the 

 results of previous investigators and claimed that the rod 

 myoid elongates in the light, thus supporting the contention 

 of van Genderen Stort. Arey (1916c) reinvestigated the 

 matter in the frog, where he found that both the red rods and 

 the green rods shorten in darkness, and concluded, '^ There- 

 fore, contrary to the conclusion of the older workers, the 

 photomechanical response of the frog's rod myoid is found 

 to be similar to that occurring in the retinas of all other in- 

 vestigated vertebrates." 



The most extensive changes in the length of the rod myoid 

 occur in fishes where, as Arey has shown, the mobility may 

 result in a 90 per cent change in length (Figures 57 and 58). 

 His observations on the movement of the so-called green 

 rods and red rods of the frog are shown in Figure 44. 



In urodeles, the rod myoid, unlike that of the cone, fails 

 to exhibit any positional changes to light. No movements 

 were observed in the gecko rods (Detwiler, 1923c), but 

 responses were found in the rods of the alligator where both 

 rods and cones occur (Laurens and Detwiler, op. cit.). 



