Another way to visualize antibody-antigen precipitation is to allow the antigen and antibody to diffuse toward one another in a semisolid medium such as agarose. This image is a top view of a thin layer of agarose in which the three visible circles represent wells punched into the agarose which are filled with either antibody or antigen solution. In this example, an antiserum prepared against elephant albumin was placed in the well at the bottom, while the other two wells contained albumin from either an elephant (on the right) or a an ancient mammoth recovered from the frozen Siberian tundra. Where the diffusing antigen and antibody meet, they form a visible line of precipitate over the course of a few hours or days.
The power of such an "Ouchterlony" gel (also known as "double diffusion in agar") is exemplified here by the presence of a small visible "spur" pointing to the left just under the "mammoth extract" well. This shows that the antibody is detecting at least two distinct epitopes on the elephant albumin, only one of which is shared by the mammoth protein. This pattern thus is one of "partial identity".