PARIETAI. EYi] AND ADJACENT OEGANS IN SFHBNODON. 147 



4. General Conclusions. 



As a result of my study of the writings of other erabryo- 

 logists_, as well as of ray personal observations, I have been led 

 to formulate the following general conclusions as to the 

 homologies and phylogenetic history of the '^epiphysis" and 

 associated organs. 



(1) The ''epiphysis'^ of Selachians is formed by a pair of 

 equally well-developed optic vesicles, originating as outgrowths 

 of the thalamencephalon, and uniting together in the middle 

 line (as shown by Locy). 



(2) In Teleosts and Amia there is also a pair of epiphysial 

 outgrowths arising in a similar way (with more or less dis- 

 placement), but the right vesicle alone gives rise to the 

 "epiphysis'^ of the adult, while the left one separates completely 

 from the brain and degenerates (as shown by Hill). 



(3) In Cyclostomes there is also a similar pair of epiphysial 

 outgrowths, which suffer displacement in such a manner that 

 the right vesicle comes to overlie the left. The right vesicle 

 forms a parietal eye, and the left one the " parapineal organ.'^ 

 These two, together with the nerve of the parietal eye, 

 constitute the "epiphysis^' (compare Ahlborn, Beard, Gaskell, 

 Studnicka). 



(4) In Sphenodon and Lacertilia the "epiphysis ""Ms a 

 composite structure in which the paraphysis and '' Zirbel- 

 polster " take a very large share, while the [)arts which corre- 

 spond to the paired epiphysial outgrowths of fishes take a very- 

 small one. These outgrowths originate, however, very much 

 as in fishes, and are subject to more or less displacement, and 

 one or other of them may give rise to a parietal eye. In 

 Sphenodon it is tlie left parietal eye which is thus developed. 



(5) The right parietal eye is represented in Sphenodon by 

 the ''parietal stalk." In Lacertilia the parietal stalk re- 

 presents either the right or left parietal eye. 



(6) The parietal eye has no real connection with the 

 parietal stalk beyond that of fellowship, and is supplied with 

 a special nerve of its own not derived from the parietal stalk. 



