Experimental design for assays of cell migration in the RMS. Two hundred-micrometer-thick sagittal slices were cut from brains dissected from embryonic day 18 or postnatal day 0–16 mice using a vibratome. Slices containing the entire RMS were selected and placed on Millicell inserts in CCM1 medium containing HEPES and 5% horse serum. Crystals of DiI were placed on the RMS using a microneedle. By selecting the position of the DiI crystals, migrating cells in every part of the RMS can be visualized. Time-lapse images of fluorescent cells were recorded over 3–10 hr using an inverted microscope.