The pros and cons of stem cell research come to the surface when we examine the third source of stem cells - embryonic cells. Embryonic stem cells are extracted directly from an embryo before the embryo's cells begin to differentiate. At this stage the embryo is referred to as a "blastocyst." There are about 100 cells in a blastocyst, a very large percentage of which are stem cells, which can be kept alive indefinitely, grown in cultures, where the stem cells continue to double in number every 2-3 days. A replicating set of stem cells from a single blastocyst is called a "stem cell line" because the genetic material all comes from the same fertilized human egg that started it. President Bush authorized federal funding for research on the 15 stem cell lines available in August 2001. Other stem cell lines are also available for research but without the coveted assistance of federal funding.