Histological changes of the digestive tract were studied in Japanese flounder(Paralichthys olivaceus)from hatching until day 50.Samples were taken after hatching,then of 5,8,15,30 and 50 days post-hatch.During the first 3 day after hatching,the digestive tract was lengthened and coexisted with the sac-yolk.The mocosae began to proliferate and differentiate.The yolk became smaller and smaller after the time of mouth opening.The liver is situated on top of the intestine.From the 15th day,they were metamorphosing.At the beginning of the metamorphosis,the digestive tract could be divided into buccal cavity,pharynx,oesophagus,post-oesophgeal swelling,intestine,and rectum.The exocine pancreas is located within the intestinal loop.Goblet cells were present in most places of the tract.Very few endocrine cells could be detected.By day 30,the posterior zone of the oesophagus had a marked increase in goblet cells density and mucosal folding.Gastric gland,columnar epithelium of the post-oesophgeal swelling allowed it to be defined as a stomach.Goblet cells also disappeared in the post-oesophgeal swelling.Pyloric caeca extended from the anterior intestine.Many endocrine cells could be found in stomach and intestine.These cells were brown staining.Some cells were round like a finger ring,and others were long,opening via a duct to the digestive lumen.By day 50,the structure of the digestive tract was mature like that of adult fish.Serotonin cells were distributed in the stomach,gastrin and P substance cells were distributed in the pyloric caeca and foregut.
