598 



THE DIGESTIVE SYSTEM 



ESOPHAGUS 



VERTICULUM 

 GE INTESTINE 



OSTflNAL GUT 



MESENCI 



CONTRIBUTED 



FROM SPLANCHNIC 



LAYERS 



OF HYPOMERE 



Fig. 278. Diagrams showing basic features of digestive-tube development in the verte- 

 brates. (A) The regions of the primitive gut where outgrowths (diverticula) normally 

 occur. (B) Basic cellular features of the gut tube. (C) Contributions of the basic 

 cellular composition to the adult structure of the digestive tract. Consult Fig. 293 for 

 actual structure of mucous layer in esophagus, stomach, and intestines. 



type, on the other hand, most of the yolk material Hes outside the confines of 

 the primitive gut tube (fig. 217), and the midgut region of the primitive tube 

 is open ventrally, the ventro-lateral walls of the tube being incomplete. The 

 latter condition is found in elasmobranch fishes, reptiles, birds, and primitive 

 mammals. In higher mammals, although yolk substance is greatly reduced, 

 the arrangement is similar to that of the latter group. The teleost fishes repre- 

 sent a condition somewhat intermediate between these two major groups. 



2. Basic Structure of the Early Metenteron (Gut Tube) 

 (Consult figs. 278A; 279A; 280A; 281A; and 282B.) 



a. Basic Regions of the Primitive Metenteron 



The primitive vertebrate metenteron possesses the following regions. 



1) Stomodaeum. The stomodaeum lies at the anterior extremity of the gut 

 tube, and represents an ectodermal contribution to the entodermal portion of 

 the primitive gut. It results from an invagination of the epidermal tube directed 

 toward the oral evagination of the foregut. The membrane, formed by the 



