This microscope photo provided by The Rockefeller University shows a human embryo 12 days after fertilization in vitro, with different cell types marked by separate colors. Scientists have gotten their first good look at a crucial stage in the development of human embryos, a step that could help clear up mysteries about the early days of a person's life. (Photo by Gist Croft, Alessia Deglincerti, Ali H. Brivanlou/The Rockefeller University via AP Photo)
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