The electrical membrane potential (Em) and electrical membrane resistance (Rm) were measured continuously during the first cleavage of Xenopus eggs, using intracellular microelectrodes. A sharp hyperpolarization of Em and decrease in Rm can be observed from 6 to 8 min after the onset of cleavage. This moment coincides with the onset of the insertion of new membrane (Bluemink and de Laat, 1973) leading to the formation of the interblastomeric membrane during normal cleavage. Removal of the vitelline membrane or exposure to cytochalasin B (CCB) leads to exposure of the entire surface area of the membrane newly formed during cleavage. These conditions allow for a direct measurement of the permeability properties of the new membrane. It was found that under these conditions Em reaches values about 3 times more negative and Rm reaches values about 1.5–3 times smaller than during normal cleavage. The extent of reduction of Rm can be correlated with the surface area of the newly formed membrane. We conclude that the new membrane has different ionic permeability properties than the pre-existing membrane (most probably a relatively high permeability for K+ ions). Its mean specific resistance is 1–2 kΩ·cm2, as against 74 kΩ·cm2 for the pre-existing membrane. No influence of CCB on the permeability properties of the pre-existing or new membrane could be detected.

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