Proliferation within cortical zones over E18–20. BrdU was administered to pregnant dams at E17. Fetuses were recovered at E18 – E21 and coronal sections were stained as described. (A) Photomontages of representative cortical sections stained with antibody to BrdU showing the progression with age of cells born on E17 from the germinal epithelium into the intermediate zone and cortical plate by E18 – E20. MZ (brown bar): marginal zone. (B) The number of BrdU positive cells, counted on 400 μm-wide ventricle-to-pia sections, in the germinal epithelium (GE, green bar), intermediate zone (IZ, blue dashed bar) and cortical plate (CP, red bar) at days E18, E19 and E21. Results for each age are from 4 separate experiments. Data are means ± SEM.