A nine-day-old human embryo seen through a microscope. “It was really, really exciting.”Human Embryo Video of a human embryo. The scientists then placed into the womb dozens of human embryos, which were left over from infertility treatments. (Institute for Bioengineering of Catalonia)The scientists were surprised by how forceful human embryos are, especially compared to mouse embryos, which appear to lie passively as a womb envelopes them. “I’m very excited to see this study,” agrees Magdalena Zernicka-Goetz , who studies human embryos at the California Institute of Technology.