PARIETAL EYE AND ADJACENT ORGANS IN SPHENODON. 145 



ig) The Epiphysis of Sphenodon and Lacertilia. 



It thus appears that the '' epiphysis '' of Sphenodon and 

 Lacertilia, as ordinarily understood, is a composite structure, 

 made up of the following constituent parts : 



(1 ) An upward arching of the thin roof of the third ventricle 

 between the superior commissure and the commissura fornicis. 

 This outgrowth arises comparatively late in development, and 

 is probably the result, at any rate to a large extent, of the com- 

 pression of the thalamencephalon between the prosencephalon 

 and the mid-brain in the straightening outof the cerebral flexure. 

 It is evidently homologous with the Zirbelpolster described, 

 for example, by Burckhardt (9) in Lacerta. It is to this part 

 that Ritter (29) proposes to restrict the term " epiphysis." 



(2) An outgrowth from the membranous roof of the pros- 

 encephalon arising in close relation with the choroid plexus 

 of the lateral ventricles, and known as the paraphysis. This 

 outgrowth grows upwards and backwards over the commissura 

 fornicis, and gives rise to a number of blind diverticula or 

 tubules, which lie beneath the parietal eye. 



(3) A number of blood-vessels, which make their way in 

 amongst the diverticula or tubules of the paraphysis. 



(4) The parietal stalk, which arises from the roof of the 

 thalamencephalon a short way in front of the optic lobes, and 

 passes upwards and forwards over the superior commissura 

 until it meets the backward-growing paraphysis. The parietal 

 stalk, however, does not appear to form any very important 

 part of the epiphysis of the adult, but is a vestigial structure 

 representing (in Sphenodon) the right parietal eye, It 

 undoubtedly gives rise to the ''epiphysial vesicle" of Ritter, 

 and to a part, at any rate, of what Spencer (33) calls the 

 " portion of the epiphysis equivalent to the pineal stalk." It 

 is clearly shown, for example, in Spencer's fig. 41, representing 

 a sagittal section through the brain of Varanus bengalensis, 

 where it is labelled Ep.^ ; and in his fig. 25, representing a 

 corresponding section of Anguis fragilis. 



This analysis of the so-called " epiphysis " into its constituent 



