In an effort to eradicate such seemingly incurable diseases as Parkinson’s, Alzheimer’s, and some forms of cancer, scientists have begun to study human pluripotent stem cells, which are thought to be the primordial cells from which a human is created. Stem cells, derived from fertilized eggs just before they would have implanted in the uterus, have the power to develop into any type of cell in the body. Because the cells have the ability to divide indefinitely outside the human body without the signs of age that afflict other cells, scientists speculate that the cells may be able to grow tissue for human organ transplants and recreate genes to combat inherited diseases.

Stem cell research has caused much controversy because scientists obtain the cells from aborted fetuses and unused embryos created at in vitro fertilization clinics. Opponents...