344 



THE EYE IN EVOLUTION 



assumes varying shajDes — a crescent above the optic disc in Rana, a 

 circle around it in Hyla and Bufo, a linear band in B. esculenta, and so 

 on (Hulke, 1864 ; Chievitz, 1891 ; Slonaker, 1897 ; von Hess, 1910). 

 The 02)fic nerve is thin and cylindrical with connective tissue septa 



Figs. 408 and 409. — The Visual Cells of Anurans. 



Fig. 408. — The dark-adapted common 

 rod (on the left) and the green 

 (Schwalbe's) rod of the leopard frog, 

 Rana pipiens (on the right) 

 (Gordon Walls). 



e, ellipsoid ; /, foot-piece ; I, ext. 

 limiliiig membrane ; m, myoid ; n, 

 nucleus ; o, outer segment. 



Fig. 401). — .Single, double and triple 

 cones from the eye of the tadpole of 

 Rana temporaria (aged 26 days). 



There is an achromatic oil-droplet in 

 the single cone, in the chief member of 

 the double cone, and not in the acces- 

 sory member of the triple cone. In the 

 double cones the accessory member 

 has an extensive paraboloid and a 

 rod-shaped outer segment. In the 

 triple cone there are 3 components, 

 2 similar in all respects to the chief 

 component of the double cone, the 

 third similar to the accessory element 

 of the double cone (L. Saxen). 



(Studnicka, 1898), while the chiasnia shows a total decussation 

 frequently in the form of large fascicula? interdigitating with one 

 another (J. Miiller, 1826 ; Leuckart, 1876 ; Gross, 1903). 



The OCULAR ADNEXA are very different from those of Fishes, for 

 in the f;; ln.lt a complicated protective and lubricating system is necessary 

 to proi ' an eye exposed to air ; lids are thus absent in the larvae of 



