PAEIETAL BYE AND ADJACENT ORGANS IN SPHENODON. l4l 



therefore requires considerable modification, I still believe that 

 there is very grave doubt as to the homology of the so-called 

 " epiphysis " throughout the Vertebrate series. I am con- 

 vinced that in Sphenodon, at any rate, the parietal stalk does 

 not give rise by itself to what is usually regarded as the epi- 

 physis of the adult, as described, for example, by HoiFmann 

 (18), who begins his description of the epiphysis of Sphe- 

 nodon thus : "Die epiphyse selbst bildet eine ziemlich weite, 

 schlauchformige Fortsetzung der dritten Hirnhohle/' 



Perhaps one of the most important facts established by my 

 investigation of the development of Sphenodon is the com- 

 plete closure of the proximal end of the parietal stalk at Stage R, 

 so that its cavity is completely shut off from that of the brain. 

 This closure seems to be effected by compression between the 

 large superior commissure immediately in front of the base of 

 the stalk, and the posterior commissure immediately behind it 

 (fig. 15). It appears to me highly probable that these two 

 commissures ultimately unite at the point where the opening 

 of the parietal stalk was placed. If this be so, the so-called 

 posterior commissure of reptiles and higher vertebrates must be 

 regarded as a composite structure, the front part of which is 

 homologous with the superior commissure of fishes. 



In any case it is clear that in Sphenodon the parietal stalk 

 cannot give rise to the main part of the epiphysis of the adult 

 as that organ is described by Hoffmann, though it may give 

 rise to a portion thereof. 



This conclusion is strongly supported by the examination 

 of other types. Ritter's observations on Phrynosoma are 

 especially instructive in this respect, and are capable of a very 

 simple interpretation in accordance with my views. Before 

 discussing these observations it may be well to quote his fol- 

 lowing paragraph (29) : 



'' In previous discussions of the nature and function of the 

 parietal organ, I believe sufficient attention has not been given 

 to the structure and development of the epiphysis and its 

 relation to the parietal vesicle, and especially its relation to 

 the so-called choroid plexus. I have designated the entire 



