WASHINGTON, Dec. 27 (UPI) -- When congressional leaders reconvene next month, they face the arduous task of clearly defining what scientists may and may not be able to do with federal dollars in the field of embryonic stem cell research.

Stem cells are unique in their ability to develop into other kinds of tissue. Many in the field believe that stems cells taken from a pre-embryo show the greatest ability to morph into other tissue and offer tremendous promise to treat a host of chronic, debilitating diseases including Parkinson's disease, Alzheimer's disease, and diabetes.

Stem cell research, however -- particularly when it involves the creation of pre-embryos -- has raised many ethical concerns. Those who oppose abortion and cloning see a stake in the debate for their issues. The debate for and against the research has drawn vehement supporters and critics to both sides.

While many who favor stem cell research supported President Bush's executive order Aug. 9 to allow federal funding for studies involving about 60 existing stem cell lines, critics question whether the order will have a chilling effect on scientific progress.

"I never understood why President Bush cut the baby in half," by permitting the research, but only on existing lines, Rep. Diana DeGette, D-Colo., told United Press International.