CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION 



529 



POSTERIOR 

 PHARYNGEAI 

 REGION OF GUT 



ENDOCAR 

 EPIMVOCARDUM 



NEURAL TUBE 



DERMOMrOTOl 



NOTOCHORD 



INTERIOR CARDINA 



ESOPHAGEA 



REGION OF G 



DORSAL MESOCARDIUM 



RT REGION 



NEURAL TUBE 

 DERMOMYOTOME 



OTOCHORO 



DORSAL AORT 



7\\\ POSTERIOR CARDiNAL 



DORSAL MESENT 



NEPHROTOME 



DORSAL PANCRE 



LIVER REGION 



LANCriNOPLEURE " 

 EPATIC DIVERTICULUW* 

 CENTRAL MESENTERY 



c. 



ABDOMINAL REGION 



BLADDER REGION 



Fig. 254. Diagrams illustrating the contributions of the mesial or splanchnic layers of 

 the hypomeres to the developing heart and gut structures in reptiles, birds, and mammals. 

 Sections are drawn through the following regions: (A) Through primitive tubular heart 

 anterior to sinus venosus. (B) Through caudal end of sinus venosus and lateral meso- 

 cardia. (C) Through liver region. (D) Through region posterior to liver. (E) 

 Through posterior trunk in region of urinary bladder. 



envelop or enclose the gut tube. This enclosure readily occurs because in this 

 region of the trunk, the gut tube lies closer to the ventral aspect of the embryo 

 than in the heart area. Consequently, a dorsal mesentery above and a ventral 

 mesentery below the primitive gut tube are formed (fig. 254C). The dorsal 

 and ventral mesenteries may not persist everywhere along the gut (fig. 254D). 

 The degree of persistence varies in different vertebrates; these variations will 

 be mentioned later (Chap. 20) when the coelomic cavities are discussed. 

 However, there is a persistence of the ventral mesentery below the stomach 

 and anterior intestinal area of all vertebrates, for here the ventral mesentery 

 (i.e., the two medial walls of the lateral plate mesoderm below the gut) con- 

 tributes to the development of the liver and the pancreas. These matters are 

 discussed in Chapter 13. 



Aside from the formation of the dorsal and ventral mesenteries by the in- 

 ward movement and fusion of the medial walls of the lateral plate mesoderm 

 above and below the primitive enteron or gut tube, that part of the medial 

 walls of the lateral plate mesoderm which envelops the primitive gut itself is 

 of great importance. This importance arises from the fact that the entoderm 

 of the gut only forms the lining tissue of the future digestive tract and its 

 various glands, such as the liver, pancreas, etc., whereas mesenchymal con- 

 tributions from the medial wall of the lateral plate mesoderm around the 



