382 



THE EYE IN EVOLUTION 



n«w.i •/■»'*'* 





c/( 





Fig. 463. — The Retin.4 of Sphesobos in the Central Area. 



Showing the shallow fovea, r, retina ; ch, choroid ; s, scleral cartil 

 ( X 90) (Gordon Walls). 



moscoiDically the fundus is reddish-broAvn witJi a stippling of golden 

 sjiots whereon the arrangement of the white and relatively coarse 

 nerve fibres is clearly delineated as they radiate uniformly outwards 

 from the optic disc. Three visual elements are present, the majority 

 of which were interpreted by the older writers as cones and are still 

 held to be such by observers such as Vilter (1951) who found a rela- 

 tionship between the receptor and 

 ganglion cells of 1 : 1, as in the lizard. 

 Walls (1934). on the other hand, 

 claimed that the prej)onderant visual 

 cells are rods with enlarged and sturdy 

 outer segments, homologous with the 

 cones of Clielonians and Crocodilians ; 

 single and double elements are present 

 in approximately equal numbers, with 

 colourless oil-droplets in the former and 

 in one component of the latter (Walls 

 and Judd, 1933). The third type of 

 cell, a small and ill- formed cone without 

 an oil-droplet, is sparse and absent from 

 the fovea (Fig. 464). The central fovea 

 is shallow but well-formed, and, if 

 Walls's interpretation is accepted, shares 

 with that of a gecko , ^ and some noctii rnal 

 primates,- the distinction of being the 

 1 p. 365. - p. 486. 



> 



Fig. 404.- 



-The Visual Cells of 



SpHEyoDOX. 



A sinalf "rod", a double "rod" and 

 a cone ( 1,000) (Gordon Walls). 



