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1. The electrical properties of ectodermal cells have been studied in embryos of the axolotl Ambystoma mexicanum between gastrulation and the closure of the neural tube.

2. At the time of neural induction by the underlying mesoderm the mean membrane potential recorded in ectoderm cells was -30 mV (± 1·5 mV S.E. of mean) and in presumptive neural cells -27 mV (± 1·6 mV S.E. of mean).

3. At late neural fold stages, when specification of the neuroectoderm is complete, the membrane potential in presumptive nerve cells was -44 mV (± 1·7 mV S.E. of mean). This is significantly greater than in cells of the surrounding ectoderm at the same developmental stage (-31 mV ± 1·5 mV S.E. of mean).

4. Current injected into an ectoderm cell spread freely throughout the neural and lateral ectoderm both before and after neural specification was complete.

5. Voltage—current relations recorded at mid-neural fold stages in the lateral ectoderm and neural plate rectified in opposite directions. In the neural plate the slope conductance rose as the internal potential was made less negative; in the lateral ectoderm the slope conductance fell with depolarization.

6. At the time of closure of the neural tube ectoderm and presumptive neural cells lose their low resistance connexions with each other. At the same time low resistance contacts are established across the mid line between ectoderm cells originally separated by the neural plate.

7. After the neural tube has closed low resistance connexions remain between presumptive neural cells, although the degree of current spread from one cell to the next is not very great.

8. The voltage—current relation recorded in neural tube cells showed a rise in slope conductance as the cell was depolarized.

9. Occasionally signs of regenerative activity were seen, but the mechanism for generating a fully fledged action potential does not differentiate until after complete closure of the neural tube.

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