Figure 5

Summary of the embryonic development of Bothrioplanida and Rhabdocoela. (A-H), schematic representations of the early development of bothrioplanids (modified from [26]) and rhabdocoels (modified from [67]). Bothrioplana lays eggs containing two oocytes and many yolk cells, which are fusing to a yolk syncytium before the egg is laid. The oocytes undergo two meiotic divisions and give rise to 8 "blastomeres" (gametes) (A), which further divide to build an embryonic blastema (B). Migrating blastema cells (C) provide hull cells enveloping the yolk syncytium and the blastema cells, which are accumulating in the brain primordium and the pharynx primordium (D). In rhabdocoels, the first cell division is equatorial, giving rise to an animal micromere and a vegetal macromere (E). Proliferation of these two initial cells forms a discoidal embryonic blastema, which is first placed in the middle of the egg (F) and later moves to one side (G), which will become the future ventral side of the embryo. The epidermis differentiates from this embryonic blastema, as do the other organs, and engulfs the mass of external yolk cells (H). In all schemes, an idealized animal-vegetal axis (ventral-dorsal axis in D, G and H) cross section of the embryo is represented (animal/ventral to the top, vegetal/dorsal to the bottom in bothrioplanids and vegetal/ventral to the bottom, animal/dorsal to the top in rhabdocoels). Yolk cells are colored in light blue, hull cells in orange and embryonic cells in gray. Drawings are not to scale. bl blastomere, "bl" "blastomeres" which are gametes, brp brain primordium, eb embryonic blastema, ep epidermis, hc hull cells, mb migrating blastomeres, pp pharynx primordium, yc yolk cell, ycn yolk cell nuclei in a yolk syncytium ys.