The Outer Subventricular Zone (OSVZ) is a germinal layer playing key roles in the development of the neocortex, with particular relevance in gyrencephalic species like human and ferret where it contains abundant basal Radial Glia Cells (bRGCs) that promote cortical expansion. Here we identify a brief period in ferret embryonic development when apical RGCs generate a burst of bRGCs that become founders of the OSVZ. After this period, bRGCs in the OSVZ proliferate and self-renew exclusively locally, thereby forming a self-sustained lineage independent from the other germinal layers. The time window for the brief period of OSVZ bRGC production is delineated by the coincident down-regulation of Cdh1 and Trnp1, and their up-regulation reduces bRGC production and prevents OSVZ seeding. This mechanism in cortical development may have key relevance in brain evolution and disease Samples were analyzed with 3 replicates of each of them (except E34SVZ that has 2 replicantes). Comparisons were done taking different reference sample depending on the comparison.