250 



THE EYE IN EVOLUTION 



THE VISUAL CELLS Constitute the most important and interesting 

 of the constituent elements of the retina. i They have been divided 

 into two types — rods and cones (Figs. 264-267). Typically the rod 

 consists of an outer and inner segment, a nucleus and a foot -piece. 

 The outer segment, possibly representing the cilium of the ancestral 



Fig. 264. 



Fig. 266. 



Figs. 264 and 265. — Typical Rods and 



Cones of the Fbog, Baxa pipiess 



(Gordon Walls). 



Fig. 264. — (a) A common rod (dark- 

 adapted) ; (6) a green rod. 



Fig. 265. — A typical cone (dark- 

 adapted). 



d, oil-droplet ; e, ellipsoid ; /, foot- 

 piece ; I, external limiting membrane ; 

 m, myoid ; n, nucleus ; o, outer segment. 



Fig. 267. 



Figs. 266 and 267. — Typical Rods and 

 Cones of Man (after Greeff). 



Fig. 266. — A typical rod. 



Fig. 267. — (a) A peripheral cone near 

 the ora ; (6) a peripheral cone near the 

 equator ; (c) a macular cone. 



o, outer segment ; 6, inner segment ; 

 c, cell fibre ; d, cell nucleus ; e, cell foot ; 

 /, ellipsoid ; g, myoid. 



ependymal cell, is the photosensitive tip of the cell ; the inner segment, 

 possibly representing the columnar body of the ependymal cell, has at 

 its outer end an ellipsoid containing mitochondria, presumably the 

 principal site of metabolic activity, while its inner end is termed the 

 myoid ....hough it is by no means always contractile. The cone has 



^ For structure of rods and cones, see C. Miiller (1926), Wislocki and Sidman 

 (1954), Sidman and Wislocki (1954) (histochemistry) ; Sjostrand (1949-53), de Robertis 

 (1956) (ele( -n-microscopy) ; Saxen (1955-6) (development) ; Sidman (1957) (phase- 

 contrast ail fractometry). 



