54 



THE VERTEBRATE RETINA 



brane, which constricts its base firmly and keeps the nucleus of the cone 

 on the vitread side. The tapered photosensitive tip of the cell is the 

 outer segment, the remainder of the cell down to the nucleus being the 

 inner segment and representing the columnar body of the ancestral epi- 

 thelioid ependymal cell. In the distal end of the inner segment lies the 



( cf 





Fig. 22 — Single cones. 



1000. 



a, of sturgeon, Acipenser fulvescens. b, of goldfish, Carassius auratus; light-adapted {i.e., 

 with myoid contraaed — cj. Fig. 62, p. 146; in fishes, the cone nucleus often lies partly or 

 wholly above the external limiting membrane, as here), c, of leopard frog, Rana pipiens: 

 dark-adapted {i.e., with myoid elongated — c/. Fig. 64, p. 148). d, of snapping turtle, 

 Chelydra serpentina, e, of marsh hawk. Circus hitdsonius; from the circumfoveal eminence, 

 f, of man; from the circumfoveal eminence. 



d- oil-droplet, embedded in: e- ellipsoid; /- foot-piece; /- external limiting membrane; 



m- myoid; n- nucleus; o- outer segment; p- paraboloid. 



ellipsoid, whose shape in the frog cone happens to justify this geomet- 

 rical name, though this is seldom true. Embedded distally in the ellipsoid 

 is the oil-droplet, which in some frog cones contains a dissolved yellow 

 pigment. The stalk-like portion of the inner segment is highly contrac- 

 tile (Chapter 7, section B) and hence is called the myoid (= muscle- 

 like). The myoid joins the large, ovoid nucleus in which the chromatin 



