One cell can be safely removed
to make an embryonic stem cell line.One kind of stem cell is controversial-- embryonic stem (ES) cells. The controversy comes from the fact that scientists have to destroy an embryo to get its stem cells. To people who view the 8-cell embryo as a life, this is not acceptable.
What if we could get ES cells without destroying the embryo? Then there wouldn't be so much controversy. In fact it would be hard to argue against using them unless someone is opposed to in vitro fertilization (IVF).
For a number of years now scientists have been able to take a cell from an embryo and test it for genetic problems. The procedure is called preimplantation genetic diagnosis or PGD.
People have this done if they are having trouble conceiving. Or if they carry in their genes some awful disease that will kill the child very early in life. (Or, more frivolously, to make sure a family has equal numbers of boys and girls-- family balancing.)
Imagine that instead of taking that cell and testing it you instead grew it into a stem cell line. Now the embryo can develop into a child. And the cell can be made into an ES cell line.