Figure 6

Summary of the embryonic development of Adiaphanida. (A-L), schematic representations of the early development of fecampiids (modified from [77]), prolecithophorans (modified from [79]) and triclads up to the incorporation of the external yolk cells by the embryo. In fecampiids, early cleavage seems not to be of the disperse type (A), as in the other adiaphanids. After cleavage, the embryo forms an open pouch (B) and incorporates inside this cavity part of the yolk cells (C). Subsequently, the embryo extends its walls to the periphery of the yolk mass, incorporating the remaining yolk and adopting a hemispherical shape (D). The yolk becomes restricted to the posterior part of the embryo, while the blastomeres in the opposite pole proliferate and form an embryonic blastema. In prolecithophorans, disperse cleavage is observed (E), although micromeres and macromeres are still recognizable. After a few cell divisions, blastomeres form an internal epidermal layer (F) that eventually covers the whole embryo and the external yolk cells after an inverse epibolic movement (G-H). The remaining blastomeres form an embryonic blastema on one side of the embryo, as observed in other neoophoran flatworms (H). In triclads, the formation of a yolk-derived syncytium where disperse cleavage takes place is observed in early embryos (I). Once a certain number of blastomeres is reached, some of them differentiate into two transitory organs (primary epidermis and embryonic pharynx (J), that will be used to ingest the maternally supplied yolk cells (K). After yolk ingestion, the remaining undifferentiated blastomeres proliferate and differentiate into the definitive organs (L), replacing the transitory ones. In all schemes, an idealized cross section of the embryo is represented. In (L), ventral to the bottom and anterior to the left. Yolk cells are colored in light blue, primary hull cells in orange and embryonic cells in gray. Drawings are not to scale. bl blastomere, eb embryonic blastema, ec epidermal cavity, epi epidermis, eph embryonic pharynx, epp embryonic pharynx primordium, es eggshell, pb polar body, pep primary epidermis, yc yolk cell, ys yolk syncytium.