The bottom of the cup shows on the exterior of the cartridge. If you look at a bare primer, you see a "Y" shaped spider that is the "anvil" the firing pin crushes the priming compound against. The compound is put in wet and dries into a thin wafer. It is generally covered by a very fine metal foil, and then the spider at the open end.
SO9
Aha... now I understand.. I always wondered, until now, why the priming compound didn't get mixed up with the gunpowder, assuming (correctly) that the primer is a cup rather than a sealed cylinder. Because the priming compound has dried, and is solid not loose, there's no problem.