Fig. 3. Expression of Ci-otx, Ci-ETR-1 and acetylated tubulin in animal cells of embryos placed in MEK inhibitor at various time points. (A-G) Anti-tubulin antibodies label epidermal sensory neurones (white arrowheads) and neurones (yellow arrows). Embryos were placed in inhibitor at the stages indicated on each panel. (E) Bright-field view of embryo placed in inhibitor at neural plate stage, showing absence of palp and pigment cell formation. (F,G) Control embryo. (H) Detection of non-epidermal tubulin-positive neurones in embryos placed in MEK inhibitor at the time points shown along the x-axis. 110+0.5H represents 30 minutes after 110-cell stage at 17°C; 110+1H represents 1 hour after 110-cell stage; neur. pl., neural plate stage, 1 hour and 45 minutes after 110-cell stage. Detection of neurones is expressed as a percentage. (C) An example of `neurones in both head and tail'. (D) An example of a `good network'. Experiments with U0126 produced similar results. (I) Embryos were cleavage-arrested at the 64-cell stage and placed in inhibitor at the time points (30 minute intervals at 17°C) shown along the x-axis. e32, early 32-cell stage; 132, late 32-cell stage. At neurula stage, embryos were scored for expression of Ci-ETR-1 and Ci-otx in a-line sensory vesicle precursors. The percentage of embryos showing expression of each marker is indicated on the graph. At least 100 embryos were scored for each time point. (J) Summary of the gradual acquisition of neural fates in a-line neural precursors. Time of application of PD184352 shown.