1. The mean resting membrane potential of cells in the neural plate of axolotl embryos increases from -25 to -45 mV (maximum values from -35 to -60 mV) when the embryos move from early to mid-neural plate stages of development. 2. Increasing the extracellular [K] to 20 mM shortly before the spontaneous increase in resting potential occurs causes neural plate cells to hyperpolarize. 3. A moderate increase in [K]omicron does not hyperpolarize the membrane of neural plate cells at earlier stages or at late neural fold stages. 4. Both the hyperpolarization produced by K and the normal increase in resting potential are blocked by cardiac glycosides. 5. Cells of the ventral ectoderm do not display an increase in resting potential during the neural fold stages of development and do not hyperpolarize in response to a moderate increase in [K]omicron. 6. The results suggest that activation of the Na pump may be an early consequence of neural induction. They are discussed in the light of this suggestion.
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