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Figure 4: Normal embryo development of the sea urchin S. intermedius: zygotes (20 min pf, Zy); blastula stage (14 hpf, Bl); gastrula stage (24 hpf, Gl); (d) prism stage (34 hpf, Pr); (e) pluteus (8 dpf, Pl). Nomarski’s optics. Bar, 50 μm.