Fig. 9

Schematic diagram of regeneration of the digestive tube. The internal structure of dissected animals and cross sections at the positions of the dotted lines are illustrated. Organs other than the mesentery and the digestive tract are ommited. a Stage I, just after evisceration. Only the mesentery (gray shaded area) and the cloaca (cl; yellow) remain in the body cavity. b Stage II. A regenerating tissue (gut rudiment) appears at the anterior side as a mass of mainly mesenchymal cells surrounded by coelomic epithelium. c Stage III. Multiple cavities are formed in the anterior regenerating tissue and these coalesce with each other to form lumens. d At stage IV when regeneration is more progressed, the gut rudiment (thickened tissue on the mesentery) becomes continuous between the anterior and posterior sides. The figure shows that the anterior and posterior lumens are not yet connected, but later a single continuous tube is completed. Intestines differentiate according to their position in the digestive tract. In the stomach, a muscle layer (not shown) develops and luminal epithelium (le) is covered with cuticles (cu), as in intact tissues (see Fig. 4a, d). ce: coelomic epithelium; cl: cloaca; cu: cuticles; elt: epithelium-like tissue; le: luminal epithelum