Stem cells are important for research and have yielded many discoveries in several areas of medicine. Adult stem cells that come from bone marrow are easily donated in procedures that help those with certain kinds of cancer, but embryonic stem cells are problematic in terms of medical ethics. In 1979 the US Dept of Health, Eduation and Welfare said that embryos could be studied and stem cells could be retrieved from them and used for up to 14 days, after which the embryos must be destroyed. That deadline was clarified later in 1984, when researchers found that it was the 14 day mark where changes in the embryos could be seen that made it more than just a clump of cells and closer to a an individual being.
Since then, the technology has advanced to where embryos can be kept viable for longer than 14 days and so the controversy still exists on what is appropriate for experimentation on cells that, for some, are considered life. These concerns are weighed against advances like the trial that showed embryonic stem cells cured induced Parkinson's disease in lab mice. While there have been no human trials as a result of that study, the problem persists in how far science can and should go in an area of research that is so complex and has such potential for both good and harmful uses.