540 LESLIE B. AREY 
The most noticeable positional changes are found in fishes, 
among which, however, the eel is exceptional, for Garten (’07) 
was unable to discover perceptible differences, although the rods 
and pigment exhibited striking movements. 
Among amphibians, the small cones of the frog (Stort) and 
of the toad (Arcoleo, ’90) give clearly defined responses when 
stimulated by light. Movements of the cones of the salamander 
were likewise described by Angelucci (90), but this was not 
supported by Garten’s (’07) observations. 
It is a matter of some theoretical importance to determine 
whether there is any correlation between the extent of move- 
ment of the cones and the ratio expressing the frequency of rods 
to the cones in various regions of the same retina or in retinas 
of different animals. In the rodless retina of Tropidonotus 
natrix, Englemann (’85) found but very limited changes in the 
length of the cone myoids, while in Testudo graeca, which 
also presumably has no rods, the evidence for movement was 
extremely doubtful. Angelucci (90), however, insisted that 
light affects the cones of Testudo marina, although to a less 
extent than it does those of the frog. The changes in the rod- 
less retinas of the lizard and the chamaeleon are also slight, as 
Chiarini (’06) and Garten (’07) respectively observed. 
In the retina of birds it is possible to compare the movements 
of the cones in regions which have many rods, few rods, and 
none at all, these regions being respectively known as the yel- 
low field, red field, and fovea. The relative change in length, 
as measured from Stort’s (’87) plates of the pigeon ’s cones, can 
be expressed as follows: 
fovea: red field: yellow field = 23 : 32 : 40. 
Garten (’07) substantiated this relation between the amount 
of movement in the fovea and yellow field of the pigeon and ex- 
tended his observations to the owl and the hen. Contrary to 
many earlier statements, the retina of the owl has been shown 
to possess exceedingly delicate cones whose movements are more 
extensive than are those of the pigeon. The rods of the hen’s 
retina are sparse and changes in the positions of the cones ap- 
parently do not oceur. 
