274 



G. R. DE BEER 



in connective tissue. The hypophysial cavity has reached 

 huge dimensions, and is now the well-known sac extending 

 back between brain and gut and communicating with the 

 exterior by the single ' nasal ' aperture. 



Text FIG. 20. Text fig. 21. 



Petromyzon planeri. 



Adult. Transverse section 

 through the pars anterior. 



Petromyzon planeri. Adult. 

 Transverse section through the pars 

 intermedia. 



Discussion. 



The hypophysial cavity of Petromyzon is remarkable for 

 several reasons. Briefly its characteristics may be considered. 

 It develops in the middle of the hypophysis tissue which was 

 a solid inpushing. This is paralleled by many other verte- 

 brates. It remains in connexion with the exterior throughout 

 adult life. This condition is only found elsewhere in Polypterus 

 and Calamoichthys. In these a persisting comiexion between 

 the hypophysial cavity and the stomodaeum is referred to by 

 Sedgwick and Wiedersheim, and it would be of great interest 

 to know its development. 



