600 THE DIGESTIVE SYSTEM 



notochord and the pre-chordal plate mesoderm. The head gut ultimately de- 

 generates. Its significance probably lies in its function as a part of the head 

 organizer. 



3) Foregut. The foregut comprises the anterior portion of the primitive 

 metenteron from the region of the stomodaeum and Seessel's pocket, posteriorly 

 to the intestinal area where arise the liver and pancreatic diverticula. It is 

 divisible into four general regions: 



( 1 ) pharyngeal area, 



(2) esophagus, 



(3) stomach, and 



(4) hepatopyloric segment. 



4) Midgut. The midgut area of the gut tube is the general region lying be- 

 tween the foregut and hindgut regions. This segment of the primitive gut even- 

 tually differentiates into the greater part of the small intestine. In the early 

 metenteron, the midgut area is concerned with the digestion of yolk material 

 in such forms as the frog or with the elaboration of the yolk sac in the shark, 

 chick, reptile, and mammalian embryos. In addition, it appears that the primi- 

 tive blood cells also are elaborated in this area. (See Chap. 17.) 



5) Hindgut. This portion of the early gut tube is located posteriorly, imme- 

 diately anterior to the proctodaeum. 



6) Tail Gut (Post-anal Gut). The tail gut represents a dorsal, posterior 

 continuation of the hindgut into the developing tail. As indicated in Chapter 

 10, it is extremely variable in the extent of its development. (Consult also 

 fig. 217.) 



7) Proctodaeum. The epidermal invagination, which meets the proctodaeal 

 or ventral evagination of the hindgut, forms the proctodaeum. The anal mem- 

 brane results when the proctodaeal inpushing meets the entodermal outpushing 

 of the hindgut. The anal membrane is double, composed of entoderm and 

 ectoderm. It is destined to disappear. 



b. Basic Cellular Units of the Primitive Metenteron 



Most of the lining tissue of the primitive metenteron is derived from the 

 entoderm of the archenteric conditions of the late gastrula. Associated with 

 the strictly entodermal portion of the primitive metenteron are two contribu- 

 tions of the epidermal tube as observed on pages 598 and 600, namely, the 

 stomodaeum and the proctodaeum. Added to this lining tissue are mesen- 

 chymal contributions, derived from the medial or splanchnic layers of the 

 hypomeric mesoderm (fig. 278B). 



The glandular structures of the digestive tube are derived as modifications 

 of the lining tissue of the stomodaeal, entodermal, and proctodaeal portions 

 of the primitive gut tube, whereas muscular and connective tissues differentiate 

 from mesenchyme (fig. 278C). 



