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1. The sensitivity of the membrane potential of cells of early embryos of Xenopus laevis to variations in the extracellular concentrations of sodium and potassium has been determined.2. Alterations in the extracellular sodium concentration have little effect on the membrane potential at all pregastrular stages tested.3. Up to the 32 cell stage an increase in the concentration of potassium in the bathing medium causes a fall in cell membrane potential only when membrane newly synthesized in the furrow during cleavage is exposed at the embryo surface, during the second half of the cell division cycle.4. Beyond the morula stage (48 cells) a fall in membrane potential on raising external potassium can only be demonstrated when the seal which isolates the intercellular fluid from the bathing medium is broken so that cells lining the inner face of the embryo come into contact with the high potassium solution.5. The results suggest that the egg membrane has little selective permeability whereas membrane synthesized after fertilization is highly potassium permeable.6. No evidence could be obtained for any potential difference between the intercellular fluid and the external bathing medium.7. Dinitrophenol, sodium azide and cyanide prevented normal development only if they were injected into the intercellular cavity. Embryos reared in solutions containing these poisons turned into normal tadpoles.8. The formation of the intercellular cavity could be halted by injecting ouabain into the cavity while it was still small. Embryos reared in ouabain turned into normal tadpoles.9. The results suggest that the active transfer of sodium ions from the cells to the intercellular spaces is an integral part of the formation of the intercellular fluid. A hypothesis for the mechanism of formation of the cavity is put forward along these lines.
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