We used an extracellular vibrating probe to measure ion currents through the cleaving Xenopus laevis egg. Measurements indicate sharp membrane heterogeneities. Current leaves the first cleavage furrow after new, unpigmented membrane is inserted. This outward current may be carried by K+ efflux. No direct involvement of the Na+,K+-ATPase in the generation of this outward current is detected at first cleavage. Inward current enters the old, pigmented membrane; however, it does not enter uniformly. The inward current is largest at the old membrane bordering the new membrane. This suggests a heterogeneous ion channel distribution within the old membrane. Experiments suggest that the inward current may be carried by Na+ influx, Ca2+ influx, and Cl- efflux. No steady currents were detected during grey crescent formation, the surface contraction waves preceding cleavage, or with groove formation at the beginning of cleavage.

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