Formation and reduction of provisional ventral foregut diverticulum in the embryos of Anguilliformes as the recapitulation of lungs

We described the development and subsequent reduction of ventral foregut diverticulum in the early ontogenesis of Anguilliformes in plankton samples collected in the coastal waters of central Vietnam. The diverticulum is a provisional structure derived from the ventral-lateral area of the caudal esophagus. It develops as a protrusion from the esophagus wall and consists of one cell layer. An increase in the volume of the diverticulum results from the proliferation of these cells and transformation of their shape. It was found that the development of the ventral diverticulum in eels is similar to the early stages of lungs formation in other vertebrates. The formation of ventral diverticulum of the esophagus was recorded in all studied embryos. The time and the extent of its development differs between species and can serve as an additional morphological character in the taxonomic identification of eels at early stages of their ontogeny.