342 THE EYE IN EVOLUTION 



marinus). The most prominent feature in the fundus is the vessels of 

 the vascular membrane. These stand out clearly, and in the main 

 vessels the large nucleated erythrocytes characteristic of Amphibians 

 can be seen clearly as brilliantly-lit points racing along after each other. 

 The arteries are thinner than the veins and lie over them ; the veins 

 are gathered into a large vena media which stands out in conspicuous 

 relief as it courses vertically downwards over or near the optic disc to 

 disappear in the ventral area of the fundus (Cuignet, 1866 ; Hirschberg, 

 1882 ; Johnson, 1927) (Plate III). 



The pigment epithelium of the retina is possessed of long processes 

 dipping down among the visual elements. 



The visual retina is avascular and has the usual vertebrate 

 architecture, the layers being of average thickness (Figs. 406-7). The 

 visual cells, however, are of unusual interest and have received much 

 study (Figs. 408-9).^ They are commonly of four types, all of them large 

 and coarse in structure : violet and green rods, single cones and double 

 visual elements, while triple visual elements have been described. The 

 violet (or red) rod, which contains rhodopsin, is unusually plump, the 

 outer segment unusually large and the nucleus in contact with the 

 external limiting membrane, a level generally occupied by the nuclei of 

 cones. The green rod (of Schwalbe) is found only in Amphibians among 

 which, however, it is widely distributed (Denton and Pirenne, 1952) ; 

 it has a smaller outer segment lacking rhodopsin, a long slim stalk, and 

 its nucleus lies at a deeper level in the inner part of the outer nuclear 

 layer ; in structure it therefore occupies an intermediate position 

 between a cone and an ordinary (red) rod (Walls, 1942). The single 

 cones resemble those of the Holosteans and Dipnoi, and in diurnal 

 types {Rana) they possess a yellow oil-droplet in the upper part of the 

 ellipsoid, a structure first described by H. Miiller (1861) and Babuchin 

 (1863-64). Double visual elements commonly occur, usually said to 

 Rana \)q " double cones " but perhaps representing the fusion of a rod and 



cone (Saxen, 1954-56) ; in these the oil -droplets are confined to the 

 main member of the pair. Triple cones, only two members of which 

 bear oil-droplets, have been described by Saxen (1953) in the retina 

 of Rana temporaria, a formation suggesting that these and the double 

 cones result from a fusion of elements rather than from a process of cell- 

 division. There is a vague area centralis which has probably more 

 resolving power than the remainder of the retina (Krause, 1875) ; it 



1 H. Miiller, 1857 ; Hulke, 1864 ; Schultze, 1866 ; Steinlin, 1868 ; Dobrowolsky, 

 1871 ; Landolt, 1871 ; Schwalbe, 1874-87 ; Krause, 1875-92 ; Hoffmann, 1876-77 ; 

 Boll, 1877 ; Kuhne, 1878 ; Dogiel, 1888 ; Cajal, 1893 ; Greeff, 1899 ; Gaupp, 1904 ; 

 Kolmer, 1904 ; Hesse, 1904 ; Garten, 1907 ; Hess, 1910 ; Arey, 1916 ; Majima, 

 1925 ; Noble, 1931 ; Rozemeyer and Stolte, 1930 ; Police, 1932 ; Detwiler, 1943 ; 

 Khau-van-Kien, 1954 ; and many others. For iiltramicroscopic structure, see Sherman, 

 1' "1 : for localization of mitochondria, see Carasso, 1954 ; for histochemistry, see 

 V- locki and Sidman, 1954 ; for development, see Saxen, 1954-56. 



