AdvertisementEmbryo Experiments Reveal Earliest Human Development, But Stir Ethical DebateMarch 02, 2017Rob SteinfacebookEmailA human embryo kept alive in the lab for 12 days begins to show signs of early development. The green cells seen here in the center would go on to form the body. This embryo is in the process of twinning, forming two small spheres out of one. (Courtesy of Gist Croft, Cecilia Pellegrini, Ali Brivanlou/Rockefeller University)