440 



THE EYE IN EVOLUTION 



Wallaby, 

 Petrogale 



are fine vessels on the optic disc, sometimes (as in the kangaroo and 

 wallaby) projecting like a dome-shaped cushion above it resembling a 

 vestigial reptilian cone.^ In those species, however, wherein the 

 choroid is under-developed (the flying-phalanger, Petaurus) or is 

 insulated from the retina by an impermeable retinal tapetum, a 

 mammalian-like retinal circulation exists, paired arteries and veins 

 radiating from the disc in the inner layers of the retina, clothed in 

 glial sheaths and protruding somewhat into the vitreous ; in the 

 opossum, Didelphys, the capillaries penetrate through the entire 

 thickness of the retina to the external limiting membrane (Plate XIII). 



Fig. 552. — The Visual Cells of an Australian Marsupial. 



The native cat, Dasyurus viverrinus. 1, outer nuclear layer ; 2, external 

 limiting membrane ; 3, visual cells, showing the filamentous rods and the 

 single and double cones with oil-droplets (in both members of the latter) 

 (O'Day). 



Koala, 

 Phascolarctus 



In the Virginian opossum, Didelphys virginiana, a retinal tapetum 

 exists, a unique phenomenon among Mammals apart from the fruit- 

 bat, Pteropus. The tapetum is in the form of a semi-circle with its 

 straight horizontal lower edge at the level of the disc ; in this area the 

 epithelial cells are tall, devoid of pigment and packed with guanine-like 

 crystals of unknown chemical nature. The visual cells are reptilian in 

 type and resemble those of the monotreme eye (O'Day, 1936-39) ; the 

 retina, in fact, is that of a Sauropsidan in the eye of a Mammal (Figs. 

 552-3). The rods are filamentous and outnumber the cones which are 

 either single or double in type, lacking paraboloids but possessing oil- 

 droplets. It is interesting that in all Australasian types so far examined 

 t:= - double cones have oil-droplets in both members ; this is a rare 



^ p. 362. Compare also the Rodents, p. 481. 



