MAIVOIALS 



483 



but in the rat, the mouse (Schwarz, 1935), the dormouse (Vilter, 1953) 

 and the guinea-pig (Kohner and Lauber, 1936 ; O'Day, 1947 ; Vilter, 

 1949), cones are present although they are very few ; according to 

 Detwiler (1949) they are absent in the chinchilla ; in the Cetacea 

 (dolphins and whales) the cones are also few or non-existent. Only in 

 the Sciuridse (squirrels,^ and particularly the marmot, the most 



i*» 



'ilHf m *.**. 







% I 



I f f 



|9 



Fig. 639. — A Mixed Rod-and-cone Placental Retina. 

 Section thioiigh tlie parafoveal part of the retina of the rhesus monkej' 

 (Mallory's triple stain, X 480) (Katharine Tansley). 



1, optic nerve fibre layer ; 2, ganglion cell layer ; 3, inner jalexiforni layer ; 

 4, inner nuclear layer ; .5, outer plexiform layer ; 6, outer nuclear layer ; 

 7, external limiting membrane ; 8, visual cells ; 9, pigmentary epithelium ; 

 10, choroid. 



diurnal of all Mammals which appears only during daylight) is a pure- 

 cone retina known to exist (Rochon-Duvigneaud, 1929 ; Karli, 1951 ; 

 Vilter. 1954).- 



The contrast between tlie different types of retinal structure in 

 Placentals is best brought out by a comparison between the rod-rich 



1 Except the nocturnal flyiiig squirrel, Pteromys. 



^ For physiological evidence based on the spectral sensitivity, see Arden and 

 Tansley, 1955 ; based on adaptation, see Tansley, 1957. 



