INTRODUCTION 597 



b. Development of the pharyngeal area 



1 ) Pharyngeal pouches and grooves 



2) Pharyngeal glands of internal secretion 



3) Other respiratory diverticula 



c. Morphogenesis and histogenesis of the esophagus and the stomach region of the 

 metenteron 



d. Morphogenesis and histogenesis of the hepato-pancreatic area 



1 ) Development of the liver rudiment 



a) Shark embryo 



b) Frog embryo 



c) Chick embryo 



d) Pig embryo 



e) Human embryo 



2) Histogenesis of the liver 



3) Development of the rudiments of the pancreas 



a) Shark embryo 



b) Frog embryo 



c) Chick embryo 



d) Pig embryo 



e) Human embryo 



4) Histogenesis of the pancreas 



e. Morphogenesis and histogenesis of the intestine 



1 ) Morphogenesis of the intestine in the fish group 



2) Morphogenesis of the intestine in amphibia, reptiles, birds, and mammals 



3) Torsion and rotation of the intestine during development 



4) Histogenesis of the intestine 



f. Differentiation of the cloaca 



C. Physiological aspects of the developing gut tube 



A. Introduction 



1. General Structure and Regions of the Early Digestive 

 Tube or Primitive Metenteron 



a. Definition 



The word metenteron is applied to the gut tube which is developed from the 

 archenteric conditions of the gastrula. The term primitive metenteron may be 

 applied to the gut tube shortly after it is formed, that is, shortly after tubulation 

 of the entoderm to form the primitive gut tube has occurred, while the word 

 metenteron, unqualified, is applicable to the tubular gut, generally, throughout 

 all stages of its development following the gastrular state. 



b. Two Main Types of the Early Metenteron 



Two types or morphological forms of early vertebrate metenterons are de- 

 veloped immediately after the gastrular stage. In one type, such as is found 

 in the frog and other amphibia, ganoids, cyclostomes, and lungfishes, the walls 

 of the gut tube are complete, and the yolk material is enclosed principally 

 within the substance of the midgut area of the tube (fig. 217). In the second 



