Fig. 8. Depiction of the hypothesis that PSR plays a crucial role in engulfing apoptotic cell corpses that affect normal embryonic development and organogenesis. The zygotic embryo is featured as a newly-fertilized entity through to the completion of the first zygotic cell cycle at zero hours, then at the cleavage stage (0.75 hpf), and cell cycles 2-7, which occur rapidly and synchronously. Embryos enter the blastrula at 2.25 hpf, at which time the metasynchronous cell cycles rapidly give way to a lengthening at the 8-10 cell-cycle stage, when the asynchronous cell cycles at the midblastula transition and epiboly commence. From 5.25 hpf, the gastrula begins development of the three germ layers, when all cells require the facility of cell movement in order to achieve their developmental goals, such as the morphogenetic movements of involution, convergence, and extension from the epiblast, hypoblast and embryonic axis through to the end of epiboly. During the early gastrula stage (6 hpf), apoptotic death can occur during the entrance to the shield stage, if the apoptotic cell corpses are not removed promptly. Inhibited removal can result from an absence of a specific engulfing receptor, such as the PSR. The accumulated corpses gradually and progressively impede the cell movement and cell-cell interaction necessary for the triggering of signaling to activate the downstream developmental events. At the onset of organ development, cells in the embryo are associated with one of three germ layers, the ectoderm, mesoderm and endoderm, from the time of segmentation (10 hpf), when somites, pharyngeal, primordia and neuromeres develop, for primary organogenesis, and for the proper appearance of the tailbud. This influences the morphogenesis of organs at the pharyngula (24 hpf), hatching (48 hpf) and early larval (72 hpf) stages.