270 



THE EYE IN EVOLUTION 



but the presence of a dendritic foot-piece in the long cells and a smooth knob in 

 the short (Tretjakoff, 1916) as well as the comjoarative and taxonomic evidence 

 collected by Walls (1935), provide weighty evidence in favour of Heinrich 

 Midler's original suggestion that, desj^ite their length, the long elements are 

 probably cones and the short, rods. At the present time, as was suggested by 

 W. Miiller (1875) and maintained by Franz (1934), it may be safer, while 

 admitting the presence of two morphologically different types of cell, to 

 refrain from dogmatic differentiation until more conclusive evidence derived 

 from their histochemistry or neural connections is available. 



The oj)tic nerve is primitive, consisting (unlike that of Fishes) of 

 non-myehnated fibres (Briiesch and Arey, 1942) ; as occurs in the 

 human embryo there is no septal system but merely an axial column 



Figs. 283 and 284. — The Optic Nerve of Cyclostomes. 



Fig. 28,3. — The optic nerve of the 

 ammoeopte larva (after Studnicka). 



Fig 



284. — The optic nerve of Lumpetra 

 fluvintili.s (after Diicker). 



In both cases there is no sejital system but merely an axial column of 

 ependymal cells running down the centre of the nerve sending processes 

 radiating to the surface. 



d, dural sheath ; pa, pia arachnoid sheath ; n, nerve fibres ; e, epen- 

 dymal cells sending out radiating processes ; oa, ophthalmic artery. 



of cell-bodies, probably ependymal in nature, running down the nerve, 

 each sending processes radiating to the sm-face forming a primitive 

 oligodendroglial system (Deyl, 1895 ; StMnicka, 1912 ; Keibel, 1928 ; 

 Walls, 1942 ; Prince, 1955) (Figs. 283 and 284). The chiasma remains 

 within the brain and in it the optic nerves cross as separate individuals 

 without division into fascicles or bundles. 



THE EXTRA-OCULAR STRUCTURES of the eye of the lamprey are 

 simple. Contrary to the configuration found in all other Vertebrates, 

 there is no skeletal orbit, but the organ lies in a simple connective - 

 tissue ca -de. The orbits and the eyes are laterally placed so that no 



