Heterochronic transplantation of E12.5 cells into E15.5 hosts showing that cells that have completed the round of cell division in the E12.5 MGE environment have adopted lower layer fates. A, Immunofluorescent detection of βgal showing six labeled cells (boxed). Arrows point to cells that are also labeled with BrdU, depicted at higher power at the bottom right panels. B, Black histograms (percentage ± SEM) indicate the layer positions of transplanted cells that span (n = 376) across multiple cortical layers. White histograms (percentage ± SEM) represent the lower layer positions of βgal+ cells (n = 34) that also stained for BrdU, indicating a final round of cell division in the donor environment. Scale bar (in A): A, 130 μm; magnified boxes, 30 μm. WM, White matter.