Images of a plate of endothelial cells growing in agar. Cells appear as dark spots and contrast is limited because introduction of stains to increase contrast could affect cell function. An early time image (top row) shows a wide trough where a lane of cells was removed by the experimenter. A later time points (bottom row) show the effect of progressive narrowing of this lane as the cells migrate to fill in the gap. Our goal is to measure the width of the "clear lane" which corresponds to the amount of cell migration in the time interval.