1. The membrane potential and permeabilities of blastomeres isolated from the ectoderm of stage 6-10 Xenopus blastulae have been investigated. The increase in membrane potential between stages 6 and 9, reported previously in intact embryos, is not clearly apparent in isolated cells. However, marked differences were observed between early and late stages. 2. The membrane specific resistance was high at all stages (100-300 k omega cm2) and increased from stage 6 to stage 9. This specific resistance is much higher than previous estimates of the permeability of newly formed membrane after fertilization and very different from values reported for differentiated cells. 3. The membrane Na-K pump activity has been measured at all stages by applying ouabain to the cells (10(-4) to 10(-3) M). The pump rate per unit surface area, calculated as the ratio of the ouabain-sensitive part of the resting potential to the specific resistance, decreased from stage 7 (about 0.19 microA/microF) to stage 9 (about 0.04 microA/microF). 4. The ouabain-insensitive part of the resting potential increased from stage 6 to 9. At all stages, the blastomeres were permeable primarily to K+; blastomeres at stage 9 were more sensitive to change of external K+ than at stage 7, suggesting an increase in K+ selectivity. 5. The membrane potential was very sensitive to external pH at all stages. External protons appeared to block the permeability to K+. At low pH, it was possible to demonstrate some permeability of early blastomeres to Na+. 6. At variable times after impalement, cells underwent an increase in K+ permeability of 5- to 10-fold. This seems to be due to ion leak from the intracellular electrode. 7. This dual membrane state was observed at all stages and it may explain some of the earlier reports of high K+ permeability.
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