714 



THE EYE IN EVOLUTION 



structures, including the skull, are sometimes transparent while occasionally 

 the degree of opacity of the integument is regulated by chromatophores 

 (Breder and Rasquin, 1950). In some cases (the trout, Salmo trutta) a 

 smaller off-shoot from the roof of the diencephalon may perhaps represent a 

 vestigial parietal organ. In the lung-fishes (Dipnoi) the pineal apparatus is 

 degenerate and makes no attempt to reach the surface or assume a sensory 

 structure. 



Figs. 856 to 859. — The Pineal and Parietal Organs in Vertebrates. 



/PI 



Fig. 856. — In Cyclostomcs (the lamprey' 



Fig. 857. — In Selachians. 

 Pa 



Fig. 858. — In Amphibians. 



Ls R -Cp 



Fig. 859. — In Rejitiles {Sj)henodon). 



A, accessory parietal body ; Cli, habenular commissure ; Cp, posterior commis- 

 sure ; H, habenular ganglion ; Ls, lamina terminalis ; n, pineal nerve ; np, jiarietal 

 nerve ; Pa, i^ai'ietal organ ; Pf, parapliysis ; PI, pineal organ ; Ps, i^ineal stalk ; 

 Pt, jDineal tract ; R, pineal recess (after Tilney). 



Among AMPHIBIANS, the primitive tailed class, Urodela (salamanders, 

 newts, Ambystoma, Proteus, etc.), possesses a very rudimentary pineal 

 organ, but the occasional possession of pigment granules (the olm, Proteus) 

 and even of some nerve fibres suggests some affinity with a photosensitive 

 structure. In the degenerate blind and limbless Csecilians (Apoda) the 

 pineal organ is similarly degenerate. In the tailless Amphibians (Anura), 

 however, it is more fully represented in the early stages of development. 

 Thus in the young frog (Rana) the pineal body comes to the surface above 

 the skull as an eye-structure, its position being indicated by a pale area 

 where the cutaneous pigment and glands are scanty or absent, but it 



