13 -THE DIGESTIVE TRACT 



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he entire digestive system of the chick embryo arises initially as a flat 

 sheet of endoderm, as shown in Figure 111. The final closed tube results 

 from an anterior and a posterior fold plus a folding in of the sides of the 

 original sheet. These folds result in the formation of, first, a pocket, the 

 foregut; next, the formation of a hindgut; thirdly, the lateral folds, which 

 bring the sides of the endodermal sheet together. In this manner a complete 

 digestive cavity is formed within the embryo. It remains open to the yolk 

 at the region of the midgut. At hatching the yolk-sac endoderrh is withdrawn 

 into the body of the chick. The relation of the formation of the gut to the 

 formation of the amnion was shown in Chapter 12. Special reference to 

 Figures 101, 103, and 104 will prove helpful. 



It must be emphasized at the start that in speaking of the differentiation 

 of such structures as tongue, lungs, intestine, etc., from endoderm we are 

 referring merely to the epithelial lining and the epithelial glands of these 

 structures. The complete organs are formed not only by endoderm but also 

 by connective tissue and blood vessels derived from mesoderm, and by 

 muscular coats which are derived from mesoderm. In addition, they are 

 innervated by nerves coming from ectoderm. Functionally speaking, however, 

 the endoderm is the most important component of the digestive system, since 



