THE PITUITARY 345 



Rathke's pouch, and is ectodermal. The beginning of the 

 pouch is in front of the oral plate. When this ruptures, its 

 upper stump, containing in its upper part the head of the 

 notochord, bends forward and narrows the mouth of the 

 epithelial pouch, leading to the formation of a definite sac 

 the hypophysial sac. The wall of the sac presses upon the 

 base of the anterior brain vesicle, giving rise at its upper ex- 

 tremity to a fold in the wall of the brain, which becomes the 

 primitive infundibulum. The primitive infundibular process 

 comprises the surrounding tissue of the tuber cinereum as 

 well as the origin of the infundibulum, and the true infundi- 

 bulum is formed at a later stage by its own growth from a 

 portion of the primitive infundibular process. The head of 

 the notochord, beyond presenting a barrier to the backward 

 growth of the sac, takes no part in the formation of the pituitary 

 body. 



Mihalkovics' work has been confirmed in the main by 

 numerous authors. Kupffer described an additional origin of 

 part of the anterior lobe of the pituitary from the entoderm 

 of the fore-gut, and this view has received support. Herring, 

 however, believes that the epithelial portion of the pituitary 

 is entirely ectodermic. The account of the last-named author 

 is briefly as follows : 



Development of the pituitary body begins very early in 

 embryonic life. In mammals the epithelial portion is derived 

 entirely from the ectodermic wall of " Rathke's pouch." Its 

 origin is single and mesial. The epithelium is early distinguish- 

 able into two parts. One of these the intermediate part is 

 closely adherent to the wall of the cerebral vesicle ; the cells 

 are clear, and tend to form colloid. The other portion of the 

 buccal epithelium gives rise to the anterior lobe proper. Its 

 cells are granular, and form solid columns separated by blood- 

 channels. 



The infundibulum is an invagination of part of the wall of 

 the thalamencephalon, which is adherent to the anterior and 

 upper wall of Rathke's pouch. It therefore possesses an epi- 

 thelial covering derived from the latter. The infundibular 

 process grows backwards, and, in the cat, retains its central 

 cavity. It is lined by ependyma cells, which, during develop- 

 ment, become elongated, so that ependyma fibres run obliquely 

 in its neck. The posterior lobe of the pituitary is, from the 



