When modeling a 3d image using a set of 2d reference images, I use the mirror modifier to speed up modeling. When I work from the center to the outside edge, I'm fine. When I work from the outside edge to the center, I either end up with a small gap right at the threshold of the mirror edge or I end up extruding the vertices until they overlap. I can't join or merge the vertices because while I'm using the mirror modifier, I can only select vertices on one side. Any help would be much appreciated.