Morphological changes in the digestive tract of the Chinese soft-shelled turtle (Pelodiscus sinensis) during embryonic development

The digestive tract development of the Pelodiscus sinensis embryo is described through the observation of the embryonic morphology on hematoxylin and eosin stained tissue sections. During the first 9 days of embryonic development, the anterior intestine of the embryo divides into the oral cavity, pharyngeal cavity, esophagus, stomach, and small intestine, while the caudal intestine differentiates into the cloaca, the anterior and caudal tubes of the large intestine. Between days 10-24, the wall of the digestive tract forms a two-layer structure consisting of mucosa and submucosa. The endoderm evolves into epithelial tissue in each part of the digestive tract, the mesoderm goes from a dense cluster of cells to looser mesenchymal tissue then divides into loose connective tissue, mesothelium, and muscle tissue. There is no clear temporal boundary between development of mesenchymal tissue and the early loose connective tissue, which is a gradual process.