PINEAL EYE IN LAGERTILIA. 22B 



Lacertilia, in which its double nature and secondary fusion 

 with the brain are quite unparalleled. At the same time it is of 

 course possible, though we have no direct evidence of the fact, 

 that the epiphysis is in a rudimentary state, and may be the 

 degenerate representative of a once well- developed pineal eye. 

 The general structure of the organ — a distal vesicular part with 

 a solid proximal stalk — being in favour of this view, as is also 

 the resemblance — upon which, however, too much stress must 

 not be laid, between the walls of the upper vesicle and those of 

 the swollen extremity of the epiphysis in Cy clod us. 



Further investigations into the structure of the epiphysis are 

 much needed amongst Pisces. At present it is known that 

 amongst Elasmobranchs the structure developes as a hollow 

 outgrowth from the roof of the thalamencephalon. This, as 

 figured by Balfour in Scyllium,^ stretches forward right over 

 the cerebral hemisphere, and comes finally to consist of (1) a 

 swollen distal extremity, and (2) a hollow stalk connecting (1) 

 with the brain roof. The swollen extremity may further remain, 

 as in Raja, external to the cranium, or become embedded 

 within the cartilage, as in Acanthias. There is thus a striking 

 similarity between this and the epiphysis at a certain stage of 

 development in Lacertilia and the final stage persistent in 

 Cyclodus. 



When, however, we come to the Amphibia we find that 

 amongst these the epiphysis passes through precisely the same 

 forms in development, but (1) remains very rudimentary indeed 

 in Urodela, and (2) after reaching a considerably higher stage 

 of development in Anura undergoes great degeneration. The 

 structure in the latter becomes differentiated into a distal 

 vesicle, connected by a solid pineal stalk with the brain ; the 

 stalk soon, however, disappears, and the distal portion lies com- 

 pletely separated extracranially, its constituent cells undergoing 

 degeneration. Never at any period does it become developed 

 in any living Amphibian into an eye. 



The word living is used and emphasised, because it is by 

 no means certain that the same remark can be applied to all 

 ' 'Comp. Embryology,' vol. ii, p. 355. 



