Then, starting immediately after re-neutralization, we took pictures to follow redistribution of the probes between differently labeled cells (a hallmark of fusion pore formation), and gradual enlargement of intercellular connections and changes in cell shapes. The white arrows in A and E indicate a cell pair where the 2 dyes are already mixing, but each of the cells remains an individualized entity. The blue arrows mark the intercellular connection that expands from A to D and from A’ to D’ when adjacent fusing cells join in a big syncytium. The pictures were taken at t = 5 min (A, A'); 25 min (B, B'); 45 min (C, C'); 105 min (D, D') after the beginning of low pH application. Scale bar, 50 μm.