Histological study of the organogenesis of the digestive system of the tonguefish, Cynoglossus semilaevis

Histological changes of the digestive system and its associated glands were studied in tonguefish Cynoglossus semilaevis from the fast day （ fast day post-hatch dph） until 30 dph. Specimens for this study were hatched from artificially spawned broodstock and maintained in the indoor cement tanks （ 20.0 - 22.0℃ ）. At mouth opening （ 3 dph）, lengthening of the digestive tract, mucosae differentiation and pancreas and liver appearance were the most apparent elements. The yolk was gradually resorbed and disappeared on 5 dph., while the digestive tract was differentiated into five portions： buccal-pharyngeal cavity, esophagus, stomach, anterior and posterior intestine. The larval digestive system was morphologically ready to process external food at this time. During the following period of independent life the most noticeable events occurring were an increase in mucosal folding, cellular differentiation in the luminal epithelia, gut segmentation and looping. Thus, these digestive tract and associated glands became mature gradually and completed the morphological digestive features characteristic with increasing age and feeding activity. Glycogen was then progressively stored in the liver. Gastric glands were observed around 23d ph, which indicates the passage through the iuvenile oeriod.