Figure 2

Summary of the embryonic development of Macrostomorpha and Polycladida. (A-I), schematic representations of the early macrostomid (modified from [43]) and polyclad development (adapted from [45, 46]). In macrostomids, early cleavage follows the typical quartet spiral cleavage pattern (A) up to the 8-cell stage, after which the four vegetal macromeres 2A-2D flatten (B) and form a yolk mantle that covers the embryo (C-D) that will be eventually replaced by the definitive epidermis. The rest of the blastomeres remain in the inner region and form an embryonic blastema from which the organs of the juvenile develop. Polyclads, on the contrary, exhibit a quite conserved quartet spiral mode of development (E-I), except that macromeres 4A-4D are smaller than the micromeres 4a-4d (H). Gastrulation occurs through epiboly of the animal micromeres over the vegetal macromeres (I). As a peculiarity of polyclad development, the macromeres 4A-4D (represented with a slashed line in H) and the micromeres 4a-4c degenerate, and thereby, the whole endoderm and a large part of the mesoderm is originated by the 4d micromere. In all schemes, an idealized animal-vegetal axis cross section of the embryo is represented (animal to the top, vegetal to the bottom), unless otherwise indicated. Yolk granules are colored in light blue, hull cells in orange and embryonic cells in gray. Drawings are not to scale. eb embryonic blastema, ec ectoderm, eym embryonic yolk mantle, ma macromere, mec mesoectoderm, men mesoendoderm, mi micromere.