At two weeks old, the human embryo has only just begun its months-long journey to become a baby. The embryo is tiny, still invisible to the naked eye. But inside it, an epic struggle plays out, as a nomadic band of cells marches toward a mysterious destiny, with the future of humanity resting on their microscopic shoulders. If you happened to have caught this show on air, you can find the second half of our broadcast version here.  This episode was reported by Molly Webster, and produced by Jad Abumrad. With scoring and original composition by Alex Overington and Dylan Keefe. Additional production by Rachael Cusick, and editing by Pat Walters. The “Ballad of the Fish” and “Gonads” was composed and sung by Majel Connery, and produced by Alex Overington. Special thanks to Ruth Lehmann and Dagmar Wilhelm. Radiolab is supported in part by Science Sandbox, a Simons Foundation initiative dedicated to engaging everyone with the process of science. And the Alfred P. Sloan Foundation, enhancing public understanding of science and technology in the modern world. More information about Sloan at www.sloan.org. Support Radiolab today at Radiolab.org/donate.