54 



PINEAL EYE 



of the lowest vertebrates extant, — fishes and amphibia. If 

 there are no suggestions of its visual nature among these 

 forms, one would be inclined to believe with O. Hertwig, 

 that the epiphysis was originally of a different function 

 and that its connection with a median eye may have been 

 altogether of a secondary character. 



The evidence as to the presence, primitively, of a median 

 eye in fishes is certainly far from satisfactory : * in all the 

 forms of recent fishes, no structure has been found associ- 

 ated with the epiphysis which, by the broadest interpreta- 

 tion, could be looked upon as suggesting a visual function. 

 It is possible that fishes and amphibia may, in their extant 

 forms, have lost all definite traces of this ancestral organ on 

 account of some peculiar condition of their aquatic living. 

 On this supposition, evidence of its presence might be 

 sought in the pineal structures of the earliest Palaeozoic 

 fishes — whose terrestrial kindred, and probable descend- 

 ants, may alone have retained the living conditions which 

 fostered its functional survival. 



It is accordingly of interest to find that in a number 

 of fossil fishes the pineal region retains an outward median 

 opening, whose shape and position suggest that it may have 

 enclosed an optic capsule. If the median eye existed in 

 these forms, it may well have been passed along in the line 

 of descent through the early amphibia (where substantial 

 traces of a parietal foramen occur, e.g. as in Cricotus) to 

 the ancestral reptiles. This view is greatly strengthened, 

 as Beard has shown, by the presence in the lamprey of a 

 pineal end organ (optic.''). 



The evidence, however, that the median opening in the 

 head shields of ancient fishes actually enclosed a pineal 



* Hertwig (Mark), Handbook of E^nbryology of Vertebrates., and Cattie, v. 

 Ref p. 250. 



