142 ARTHUR BENDY. 



structure found in connection with tlie roof of the thalamen- 

 cephalon as the epiphysis ; but, as already said, I have con- 

 siderable doubt as to the wisdom of so doing. For the sake of 

 precision it would seem best that the term epiphysis should be 

 limited to the structure which arises as an evagination from 

 this portion of the brain. Certain it is that the large blood 

 sinus which I have described as a part of the epiphysis in 

 Phrynosoma cannot be regarded as forming an essential 

 portion of the structure^ and I think it quite possible that 

 what I have called the epiphysial vesicle is not a portion of 

 the epiphysis, should the term be limited as I have suggested 

 that it ought to be. The distinctness of the epiphysial vesicle 

 from the proximal portion of the epiphysis in the adult 

 Phrynosoma is without exception, so far as my observations 

 have gone ; and if it is regarded as having been derived from 

 the epiphysis, then we have two vesicles instead of one that 

 have arisen in this way, and the difficulty of explaining the 

 nature and function of the whole structure is correspondingly 

 increased." 



Now if we limit the term epiphysis as I understand B,itter 

 to suggest, we shall limit it to the comparatively late evagina- 

 tion of the thin roof of the third ventricle between the parietal 

 stalk and the paraphysis. There appears to me to be a good 

 deal in favour of this definition of the epiphysis; but if we 

 accept it, then it is quite clear that neither the parietal stalk 

 nor the parietal eye has anything whatever to do with the 

 epiphysis, but both develop much earlier, and also behind it. 

 It is also obvious that the epiphysis in Ritter^s sense is not 

 homologous with the epiphysis of fishes as described by Hill. 



Tiie parietal stalk in Phrynosoma is undoubtedly repre- 

 sented by what Hitter terms the '' epiphysial vesicle," whose 

 cavity, as in Sphenodon, is completely shut off from that of 

 the brain. 



In comparing Ritter's fig. 9, representing a sagittal section 

 through the thalamencephalon and adjacent structures in 

 Phrynosoma, with my fig. 15, representing a corresponding 

 section of an advanced embryo of Sphenodon, it is necessary 



