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Department of Zoology, University of Washington, Seattle 98195.

1. The whole-cell variation of the patch clamp was used to study ion channel properties in the unfertilized oocyte, and in surgically isolated blastomeres from 2-, 4-, and 8-cell embryos of the ascidian, Boltenia villosa. 2. The unfertilized oocyte has three major voltage-dependent currents: a transient, inward Na+ current; a transient, inward Ca2+ current; and an inwardly rectifying K+ current. 3. The total surface area of the embryo, either measured by capacitance or calculated from cell diameters, increased about 2.5-fold between fertilization and the 8-cell stage. 4. The Na+ current almost completely disappeared from the embryo by the time of first cleavage and was undetectable in any of the blastomeres at the 8-cell stage. This loss was too large to be explained by the dilution of channels in the oocyte due to newly added membrane. 5. Both the Ca2+ current and the inwardly rectifying K+ current were maintained at constant or slightly increased density through the first three cleavage cycles. This suggests that these channels are added along with new membrane during these stages. 6. No differences in mean current densities of blastomeres of different developmental fates were detected through the 8-cell stage. 7. Continuous recordings in single egg cells between fertilization and first cleavage, using two-microelectrode voltage clamp, revealed the increase in capacitance, Ca2+ current amplitude, and K+ current amplitude, and the loss of Na+ current predicted from the blastomere studies.

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