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Fig. 10 Nocodazole, a microtubule-depolymerizing agent, mimics the effect of UV. (A) An untreated embryo at 50% epiboly. A sibling, incubated in 1 μg/ml nocodazole, has reached only 30% epiboly (B). (C,D) Observations made at a later stage, when controls have just completed epiboly. 2 μg/ml of nocodazole caused the blastoderm to pinch off the yolk cell (C), while 1 μg/ml caused severe retardation of epiboly (D). At 12 hours after fertilization, when untreated embryos have long finished epiboly (E), an embryo treated with 0.6 μg/ml has still not closed the blastopore (F, arrowhead). Although epiboly is retarded, a distinct axis has formed, with somitogenesis well underway.