The natural history of the subependymal layer around the lateral ventricle of the mouse brain was studied from its appearance at E11 up to 22 months postnatum. In the young embryo four regions of the ventricle can be recognized by their histological characteristics: (1) the ventricular roof, (2) the medial roof, (3) the ventricular elevations and (4) the medial wall. The characteristics of the ventricular roof and ventricular elevations were examined in detail. The ventricular roof appears to be the main site of production of cortical neurons while the subependymal layer of the ventricular elevations seems to be the main site of origin of forebrain glia. The age of differentiation of the ependyma differs for each region, with the medial roof differentiating first, followed by the ventricular roof and medial wall, and ventricular elevations or lateral wall last. Differentiation begins with a change from pseudostratified columnar epithelium to simple columnar epithelium and the appearance of cilia in large numbers.