Purified P7 or E18 precursor cells, or E18 precursor cells that were passaged after 10 d in culture, were cultured as in Fig. 4 at low density and followed by time-lapse video recording. The migration rates during two 4-h periods between days one and two in culture were calculated for 7 E18 cells, 14 P7 cells, and 13 E18 + 10 DIV cells. The average migration rates are expressed as mean ± SEM.