170 



Herring 



no par-t niechanicall}^ or otherwise in the formation of the pituitary. Its 

 situation, nevertheless, is not without significance ; its arrest behind the 

 anterior end of the brain tube allows the latter to communicate with the buccal 

 epithelium and possibly with the fore-gut in some animals. Had the noto- 

 chord grown further forward, a median origin for the anterior lobe of the 



Fig. 4. — Mesial sagittal sectiuii through part of head of an 18-mm. kitten. 



a, hypophysial sac now closed below ; 6, infundibular process ; g, neck of sac connected with nasal 

 mucous membrane ; h, cartilage of sphenoid bone. 



pituitary would have been impossible. The median origin of the pituitary, 

 or rather the ancestral condition which this implies, may indeed explain 

 why the head of the notochord has been arrested in this situation. 



In an 18-mm. cat embryo considerable changes have taken place (fig. 4). 

 Rathke's pouch has become a closed sac, but its wall is still connected by 

 a stalk of epithelium with what is now becoming nasal mucous membrane. 

 The narrowing of the neck of the sac, its closure and ultimate disappear- 



