A 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 hide caption

Four sheep cloned from the same genetic material as Dolly roam the paddocks in Nottingham, England. The University of Nottingham hide caption

Ken (left) and Henry were created using DNA plucked from a skin cell of Melvin, the beloved pet of Paula and Phillip Dupont of Lafayette, La. Edmund D. Fountain for NPR hide caption

In 1954, Dr. Frederick C. Robbins, then chief of pediatrics and contagious diseases at Cleveland Metropolitan General Hospital, was one of three winners of that year's Nobel Prize in medicine. The scientists' work, which led to a vaccine against polio, was performed in human fetal cells. AP hide caption

Ryoji Noyori, a Nobel Prize-winning chemist and president of Japan's prestigious RIKEN research institute, bows at a news conference in Tokyo Tuesday to apologize for the scientific misconduct of a RIKEN colleague. Eugene Hoshiko/AP hide caption

A mouse embryo grows from stem cells made by stressing blood cells with acid. The blood cells are tagged with a protein that creates green light. Courtesy of Haruko Obokata hide caption

After President Obama overturned Bush-era policy restricting federal funding of embryonic stem cell research in 2009, Nebraska Right to Life led a protest of the research outside the University of Nebraska regents' meeting. Nati Harnik/AP hide caption

Human embryos grow in a petri dish two days after scientists in Oregon cloned them from a donor's skin cell. http://www.flickr.com/photos/ohsunews/8726915230/in/photostream//Courtesy of OHSU Photos hide caption

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embryonic stem cells : NPR