268 



G. R. DE BEER 



thickens and its cells become ovoid and glandular and less 

 closely packed. Dorsally this thickening is for the greater part 

 of its length closely adpressed to the floor of the infundibulum. 

 The layer of neuroglia which separates the cells lining the 

 infundibular cavity from the outer membrane of the brain- 



Text-fig. 11. 



P.E 



Petromyzon fluviatilis. Ammocoete. Transverse 

 section just posterior to the apex of the external depression 

 showing absence of hypophysial cavity. 



wall has also thickened. All this time the apex of the hypo- 

 physis depression has not moved, and the differentiation of the 

 glandular hypophysis takes place from a solid strand of cells, 

 there being still no true hypophysial cavity. 



In the Ammocoete a cavity appears in the region just anterior 

 to the gland. It has no connexion with the external depression 



