INTRODUCTION 601 



3. Areas of the Primitive Metenteron from which 

 evaginations (diverticula) normally arise 



Certain areas of the primitive metenteron tend to produce outgrowths (evag- 

 inations; diverticula). The following comprise these areas (fig. 278A). 



a. Stomodaeum 



In the middorsal area of the stomodaeum, a sac-like diverticulum or Rathke's 

 pouch, invaginates dorsally toward the infundibulum of the diencephalic por- 

 tion of the brain. It remains open for a time and thus retains its connection 

 with the oral epithelium. Later, however, it loses its connection with the oral 

 cavity and becomes firmly attached to the infundibulum of the brain. It even- 

 tually forms the anterior lobe of the hypophysis or pituitary gland. (See 

 chapters 1, 2, and 21.) Other diverticula of the oral (stomodaeal) cavity 

 occur. These evaginations form the rudiment of the oral glands and will be 

 discussed on page 617. 



h. Pharynx 



The pharyngeal area or pharynx represents the anterior portion of the fore- 

 gut, interposed between the stomodaeum or oral cavity and the esophagus. 

 This general region has four main functions: 



(1) external respiration, 



(2) food passage (alimentation), 



(3) endocrine-gland formation, and 



(4) development of buoyancy structures. 



In most vertebrates, five or six pairs of lateral outgrowths, known as the 

 visceral or branchial pouches are formed. A ventral outpocketing or outpocket- 

 ings also occur in all vertebrates. The thyroid-gland diverticulum is the most 

 constantly formed ventral outgrowth, but lung and air-bladder evaginations 

 are conspicuous in most vertebrate species. Dorsal and dorso-lateral air- 

 bladder evaginations occur in many fishes. 



c. Anterior Intestinal or Pyloric Area 



The anterior intestinal area of the primitive gut, immediately caudal to the 

 stomach region, is characterized by a tendency to form diverticula. Various 

 types of outgrowths occur here, the most constant of which are the hepatic 

 (liver) and the pancreatic evaginations. In lower vertebrates, such as teleost, 

 ganoid, and some elasmobranch fishes, blind digestive pockets, the pyloric 

 ceca, may be formed in this area. 



d. Junction of Midgut and Hindgut 



At the junction of the developing small and large intestin s, outgrowths are 

 common in many of the higher vertebrates. The diverticula which occur here 



