At S14, the wall of the hemispheres consists of the ventricular zone (VZ) and the marginal zone (MZ), which is still free of cells. The first young neurons leaving the VZ form the intermediate zone (IZ), which is mainly made up of neurons and of axons running tangentially. At S21, towards the end of the embryonic period, young neurons migrate through the IZ guided by the radial glia and move into the marginal zone. They form a plate of closely-packed cells called the cortical plate (CP). The neurons migrating later generally settle down outside the cells already present. Through these means, the cells that migrated first lie inside and those which migrated later are on the outside; this process is known as "inside-out-layering". The CP subdivides the MZ into an external layer (later cortical layer I) and an internal layer (subplate (SP)). Cell proliferation gradually decreases in the VZ. However, from S17 onwards it continues for quite some time in the immediately neighbouring zone, the subventricular zone (SZ). The CP continues its differentiation and gives rise to the cortical layers II to VI (from week 25 up to birth). The proliferation in the VZ and the SZ decreases. During the month following birth, the SP dissolves. The majority of its cells degenerate; others are thought to migrate together with their afferents into the cortical layers or else remain in the white matter (WM) as interstitial neurons. The IZ and the SZ are replaced by nerve fibres of the white matter and the VZ is replaced by the ependyma covering the ventricle.