VISUAL SYSTEM: STATE OF THE ART 



55 





,• 



I 



- 



•  - » 



Figure 10 Electron micrograph of a cone outer segment in the retina of Car- 

 charodon carcharias. The electron dense layers represent the lipid-rich receptor disks 

 which contain the visual pigment. Infolding of the plasma membrane is clearly shown 

 at the bottom of the inset. It is this infolding which distinguishes the cone outer 

 segment from that of rods. Original magnification 12 000X. Cone disks are approx. 

 3 jum in width. (From Gruber and Gulley, unpublished observation.) 



the cone nuclei straddle the external limiting membrane (ELM) while the rod 

 nuclei of this shark occur well below the ELM. 



According to Cohen (1972), the shape and location of photoreceptors 

 suggest that they evolved from ciliated, ependymal-like cells. Ependymal 



