90 VERTEBRATE PHOTORECEPTORS 



there occurs a heavy zone of pigment which almost com- 

 pletely enshrouds the outer segments (Figure 41). This 

 seems to be in marked contrast with all other nocturnal 

 forms. One can hardly see the reason for the heavily pig- 

 mented retina of this form unless it be to absorb refracted 

 light. In the gecko, as in other reptilian forms with only 

 one type of visual cell, sUght migration of the pigment occurs. 

 Such meager migration would seem to have no adaptive 

 value as was postulated by Exner and Januschke for eyes 

 with both rods and cones (e.g. alligators). Garten's theory 

 necessitates the assumption that there is no need for pigment 

 migration in an eye containing exclusively cones or rods. 

 Despite this, however, moderate migration has been demon- 

 strated in the retinas of reptilian forms possessing cones only. 

 If the pigment is of value in absorbing refracted light and 

 thus enhances independent stimulation of the individual 

 cells, it would seem to be more important in eyes with 

 cones — particularly those with large ellipsoids, paraboloids, 

 and oil drops, which are supposed to be highly refractive. 

 Lateral losses from the cones of mammals, therefore, should 

 be less serious because they lack oil drops and paraboloids. 

 Fishes which have abundant migratory pigment also lack 

 paraboloids, but here the ellipsoid by reason of its shape 

 may be more highly refractive. It is interesting to note 

 in this respect that pigment is very much heavier in fishes 

 than in mammals. The evidence for a correlation between 

 the amount of pigment and the refractivity of the visual cells 

 is not conclusive, but seems worthy of investigation. It 

 would seem not so important to isolate the individual rods, 

 for rods apparently need all the available light as evidenced 

 by the presence of a reflecting tapetum and the absence of 

 pigment in many nocturnal forms. Cones, for the sake of 

 more efficient visual acuity, may require more thorough 

 optical isolation by the pigment. 



Despite all the theoretical considerations, however, there 

 exist so many disparities, that an all-inclusive theory does 

 not seem to cover all the conditions. 



