Development of the cortical zones between E17–20. False-coloured photomontages of H&E stained coronal sections across the thickness of the cortical mantle from the brains of E17-E20 Wistar rat pups with the ventricular surface at the lower edge. Montages were selected as representative specimens from at least 3 separate pups from different litters. At E17 there is no recognizable cortical plate while at E18 it is well developed. At E18 and E19 there is a zone above the GE (marked with a vertical line) that may represent a concentration of cells produced from the proliferation occurring in the GE, which have yet to migrate. The GE decreases and the IZ and CP increase in thickness with age. GE: Germinal epithelium; IZ: intermediate zone; CP cortical plate; MZ: marginal zone.