Stem cells are cells that can divide for an infinite period of time when being grown outside of the body, and which can differentiate into various types of specialized cells. When fertilization of an egg with sperm occurs, the resulting fertilized cell has the capability to form an entire organism. The cell is described as being totipotent (having total potential). After some time, as rounds of cell division occur, specialization of cells occurs. But, early in fetal development, before the developing mass of cells attaches itself to the wall of the uterus, some cells still retain the ability to form virtually every type of cell in the body. These cells are pluripotent (capable of differentiating into many types of cells but not all types required for fetal development). With continued fetal development, further specialization of pluripotent stem cells results in multipotent stem cells--cells that...