Well, in most cases cellular determination is due to a process known as inductive signaling that takes place between cells. Now, during inductive signaling one cell produces a special type of inductive signal, a molecule known as a ligand. And that ligand moves on to a second cell and it influences the second cell to basically choose that major to begin following a specific type of instruction set of instructions, that is cell differentiation to basically produce a special type of cell. Now, we have three mechanisms of inductive signaling. We have one called diffusion, one called direct contact and the other one called gap junctions. So let's begin with diffusion.