Many cells rely on chemotaxis -- detecting differences in concentrations of chemicals on one side compared to the other -- in order to determine which way to move. Our neutrophils (white blood cells) use this to find and destroy invading bacteria as a part of our immune system. The video at the link on Blackboard (also below) shows chemotaxis of amoeboid cells. In other words, it shows cells migrating toward a region with a high concentration of a chemical, in this case cAMP. Cells start in a large reservoir on top (outside of the field of view) and migrate toward a large reservoir filled with a high concentration of a chemical that attracts the migrating cells (chemoattractant) which is below the field of view. The point of view is looking down on a horizontal surface so the cells are not "falling" but moving on their own to find the chemoattractant.