112 VERTEBRATE PHOTORECEPTORS 



but they are not localized and are interpreted as due to 

 shrinkage resulting from technical procedures. It would 

 seem impossible to think of a locahzed convolution of the 

 retina as a region for increased visual acuity, representing 

 the forerunner of the macula and fovea. 



The development of foveas (and they are always associated 

 with macular areas as generally defined) has no absolute 

 dependence upon any particular arrangement of opticus 

 fibers. It is true that in humans, the fovea develops in the 

 line where the temporal and nasal fibers of the retina meet 

 (the nasal fibers crossing — the temporal fibers remaining 

 uncrossed). However, a very prominent fovea is present 

 in diurnal lacertilians where there is complete crossing of 

 the optic tracts, and in many salt-water fishes where there 

 is also complete decussation, there is present a fovea lateralis. 

 How could Elliott-Smith's interpretation apply to birds 

 where in many forms there are two maculae and two foveae? 

 The opticus fibers, even were they both crossed and uncrossed, 

 could not possibly meet in two widely separated places. 

 Regardless of the position of the eyes in birds, optic decussa- 

 tion in these forms is also complete (Harris, 1904). Amphibia 

 with complete optic decussation lack a fovea. It is apparent, 

 therefore, that the development of this structure is not 

 necessarily bound up with any particular arrangement of 

 crossed and uncrossed opticus fibers. 



According to Harris (op. cit.) optic decussation is complete 

 in all fishes, amphibia, reptiles, and birds, whether they 

 possess binocular vision or not. He regards binocular vision 

 as originally associated with carnivorous habits — pointing 

 out its existence in a few sharks and rays, in toads, and in 

 many carnivorous birds (e.g. the larger gulls, some penguins, 

 hawks, owls, and vultures) . Among mammals it is especially 

 developed in carnivora and in the primates. Thus while 

 many animals below the higher mammals enjoy a fair degree 

 of binocular vision, their optic nerves show complete decus- 

 sation and their vision is thus not stereoscopic in the same 

 sense as in higher mammals, anthropoids, and man, where 



