In this video you can see an invidual deep cell migrating through the yolk sac of the killifish embryo. How do these cells crawl? They repeat a cycle of extending large protrusions at the leading edge, using that protrusion to pull forward, and then letting go at the rear end of the cell. This depends on rapid lengthening and shortening of actin filaments, as new subunits are added near the front of the cell, and filaments are depolymerized farther back. In this video you can see a single deep cell changing direction. Because the embryo was injected with a GFP-actin plasmid construct just after fertilization, some cells are synthesizing fluorescent actin, which you see as the brightly glowing protein inside the cell. A concentrated patch of actin appears near the new front end, before any membrane is extended.