The embryonic development of the rhabdocoel flatworm Mesostoma lingua (Abildgaard, 1789)

The embryonic development of the flatworm Mesostoma lingua was studied usin g a combination of life observation and histological analysis of wholemount preparations and sections (viewed by both light and electron microscopy.) We introduce a series of stages defined by easily recognizable morphologica l criteria. These stages are also applicable to other platyhelminth taxa th at are currently under investigation in our laboratory. During cleavage (st ages 1 and 2), the embryo is located in the center of the egg, surrounded b y a layer of yolk cells. After cleavage, the embryo forms a solid, disc-sha ped cell cluster. During stage 3, the embryo mi grates to the periphery of the egg and acquires bilateral symmetry. The side where it contacts the egg surface corresponds to the future ventral surface of the embryo. Stage 4 i s the emergence of the first organ primordia, the brain and pharynx. Gastru lation, as usually defined by the appearance of germ layers, does not exist in Mesostoma; instead, organ primordia emerge "in situ" from a mesenchymal mass of cells. Organogenesis takes place during stages 5 and 6. Cells at t he ventral surface form the epidermal epithelium; inner cells differentiate into neurons, somatic and pharyngeal muscle cells, as well as the pharynge al and protonephridial (excretory) epithelium. A junctional complex, consis ting initially of small septate junctions, followed later by a more apicall y located zonula adherens, is formed in all epithelial tissues at stage 6. Beginning towards the end of stage 6 and continuing throughout stages 7 and 8, cytodifferentiation of the different organ systems takes place. Stage 7 is characterized by the appearance of eye pigmentation, brain condensation and spindle-shaped myocytes. Stage 8 describes the fully dorsally closed a nd differentiated embryo. Muscular contraction moves the body in the egg sh ell. We discuss Mesostoma embryogenesis in comparison to other animal phyla . Particular attention is given to the apparent absence of gastrulation and the formation of the epithelial junctional complex.