Heterochronic transplantation of E12.5 cells into E15.5 hosts showing cells that have undergone the last mitotic division in the older environment have adopted upper layer fates. A, D, Immunofluorescence for βgal showing the location of transplanted cells (boxed) in multiple cortical layers. The presence or absence of BrdU in each of these individual cells is depicted in higher-power views at the bottom right. Arrows in D point to double-labeled cells that are distributed in upper cortical layers. B, The same section in A showing upper layer distribution for BrdU-labeled host cells after exposure to BrdU at E15.5. C, 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 upper layer positions ofβgal+ cells (n = 30) that are also stained for BrdU, indicating a final round of cell division in the host environment. Boxed insets showβgal+ cells at higher magnification, with arrows indicating double labeling for BrdU. Scale bar (in A): A, B, D, 150 μm; magnified boxes, 30 μm. WM, White matter.