Embryos during late blastula and early gastrulation phases. (A) 128-cell stage (5 h); (B) early high stage (8 h); (C) late high stage (10 h); (D) sphere stage (14 h); (E) dome stage (15 h); (F) 15% epiboly (16 h); (G) 20% epiboly (18 h). (H) 30% epiboly (20 h). The position of the germ ring (gr in F-H) is indicated by the dashed lines. Abbreviations: b, blastodisc; bd, blastoderm; es, embryonic shield, ez, evacuation zone, gr, germ ring; ps, perivitelline space. Scale bar = 500 μm.