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Stages 1–4: All embryos oriented with anterior region at top of image. Scale bar = 1 mm. Stage 1 (0–20 h). Stage 2 (20–22 h). Stage 3 (22 h). Stage 4 (24 h). Abbreviations: af, amniotic fold; am, anterior margin of mesoderm; ao, area opaca; ap, area pellucida; es, embryonic shield; pf, primitive fold, lateral lip; pg, primitive groove, blastopore; ps, primitive streak; ta, thickened area; zj, zone of junction.
