PINEAL EYE IN LACERTILIA. 219 



formed cavity, the primary optic cavity which persists in the 

 pineal eye entirely disappearing in the paired eyes. 



There is, however, this difference, that in the pineal eye the 

 posterior portion of the vesicle wall forms the retina, in the 

 paired eyes the anterior ; the lens of the pineal eye being a 

 structure totally distinct from that of the paired eyes. 



In the pineal eye both light transmitting and 

 light receiving structures are formed out of the 

 walls of the neural canal; the absence of this in the 

 paired eye does not perhaps constitute so great a difference as 

 appears at first sight, for though the lens is not formed out of 

 the neural wall it is formed out of epiblast cells exactly as this 

 is, and in such forms as the Amphibia, where the epiblast is 

 divided into two layers, nervous and epidermic, then the lens 

 is formed solely by the cells of the nervous layer. 



In both cases, finally, the nerve-fibres are in connection 

 with the external lying elements and retain this connection 

 throughout life, to do which, after invagination has taken 

 place in the paired eyes, they must pierce the walls of the 

 secondary vesicle ; there is thus produced the phenomenon of 

 the nerve-fibres spreading out " in front of " (as it is called), 

 and internal to, the retinal elements, though, morphologically 

 speaking, they are behind and external to them, exactly as in the 

 pineal eye. 



Significance of the Organ. 



In all forms of Vertebrates the epiphysis arises at an early 

 stage as a hollow outgrowth from the roof of the thalamen- 

 cephalon. Goette stated that the epiphysis was identical in posi- 

 tion with the anterior neuropore — the part at which the walls of 

 the neural canal remained longest in connection with the epiblast 

 — but there seems to be no doubt whatever that this is not the 

 case and that the rudiment of the epiphysis is formed at an early 

 period in a position some little way posterior to that of the an- 

 terior neuropore. There can thus be no connection between 

 the persistent anterior neuropore of Amp hiox us and the epi- 

 physis of other animals, supposing the former to be equivalent 

 to the neuropore of remaining Chordata, which cannot be 



VOL. XXVII, PART 2. NEW SER. Q 



