128 THE RUDIMENTARY MEDIAN EYE. 



surface, as in the lateral eyes of all vertebrates, but, on 

 the contrary, is distributed over its exterior surface. It 

 is, therefore, as De Graaf pointed out, formed in this 

 respect on the type of the usual invertebrate eye ; so 

 that we have the remarkable fact that in the same 



m, 



CM 



E.p 



OpL 



Fig. 82.— Diagram of a section through the skull and pineal eye of Lacerta viridis. 

 C, Cuticle ; Pa, parietal bone ; Ep, epidermis ; L, lens ; Pig, Pigment ; E, rete 

 muscosum ; Cff, cerebral hemisphere ; N, nerve ; E.p, epiphysis ; OpL, optic lobe 

 of brain. 



vertebrate animal we find eyes formed on two different 

 types. Not only so, but the development is dissimilar, 

 the lens of the pineal eye being formed out of the 

 walls of the neural canal. So that the lens of the 

 pineal eye is a totally different structure from that of 

 the lateral eyes. 



Spencer observed no effect whatever when he threw 

 a strong light on the pineal eye. In fact, he does not 

 believe that in any of the species examined by him 

 the organ is at present in a functional condition. 

 Indeed, in some cases the cornea is quite opaque, and 

 in others the nerve to the brain is not continuous ; so 

 that there can be no vision. At the same time, it 

 seems to be established that this organ is the degraded 

 relic of what was once a true eye. 



From the size of the pineal orifice in the skull of 



