Figure 4

Summary of the embryonic development of Lecithoepitheliata and Proseriata. (A-H), schematic representations of the early development of lecithoepitheliates and proseriates (both modified from [29]). Lecithoepitheliates exhibit regular quartet spiral cleavage (A) and gastrulate by epiboly of the micromeres over the vegetal macromeres (B). During gastrulation, however, the micromeres 2a-2d and 3a-3d at the edge of the blastopore differentiate into hull cells, which engulf a portion of the yolk (in X. steinöcki, C) or the whole portion of maternally supplied vitellocytes (in G. applanata). The inner mass of blastomeres differentiates into an embryonic blastema that occupies the future ventral side of the embryo, and in X. steinböcki a second hull membrane is formed to incorporate the remaining yolk cells inside the eggshell (D). In proseriates, quartet spiral cleavage is only observed up to the 8-cell stage (E). After that, the embryo develops first into a coelogastrula (F) and later into a compact discoidal stereoblastula in which 6 peripheral blastomeres differentiate into a hull membrane that engulfs the yolk cells (G). As in lecithoepitheliates, the inner blastomeres form a discoidal embryonic blastema that occupies the future ventral side of the embryo (H). In all schemes, an idealized animal-vegetal axis (ventral-dorsal axis in D and H) cross section of the embryo is represented (animal/ventral to the top and vegetal/dorsal to the bottom). Yolk cells are colored in light blue, primary hull cells in orange, secondary hull cells in green and embryonic cells in gray. Drawings are not to scale. bl blastomere, bp blastopore, eb embryonic blastema, ec ectoderm, en endoderm, fhm first hull membrane, hm hull membrane, ma macromere, me mesoderm, mi micromere, hc hull cells, phc primary hull cells, pm primary mesoderm, shm second hull membrane, yc yolk cell.