The digestive tract of the vertebrates consists of endodermal epithelium and mesenchyme derived from splanchnic mesoderm. In birds and mammals, the initially flat endodermal sheet forms a tubular pocket by folding and fusion of left and right sheets at the midline. The endodermal tube becomes surrounded by mesodermal cells, which later differentiate into the mesenchymal part of the tract and finally into connective tissue and muscles.

Regional differences of digestive tract are recognizable soon after the establishment of a tubular tract by the form and the position of digestive organ rudiments, which vary from one organ to another and from one developmental stage to another. Inside each organ, morphological and functional differentiation proceeds by the cooperation of the two components, the endodermal epithelium and the mesenchyme (28, 61). In general, the vertebrate digestive tract becomes divided anteroposteriorly into the esophagus, stomach, small intestine, cecum, large intestine and allantois. In birds, the stomach is further divided into the proventriculus (glandular stomach) and gizzard (muscular stomach). The liver and pancreas develop from the primordia formed in the small intestine.