Some mammals and birds independently evolved an enlarged telencephalon. They appear to have done so, at least in part, by developing a thick telencephalic subventricular zone (SVZ). We suggest that this correlation between telencephalic enlargement and SVZ expansion is due to a mechanical constraint acting on the proliferative ventricular zone (VZ). Essentially, we argue that rapid proliferation in the VZ after post-mitotic cells in the overlying mantle zone have begun to form limits the VZ's tangential expandability and forces some proliferating cells to emigrate from the VZ and expand the pool of proliferating cells that comprise the SVZ.