A researcher who created human embryos with both male and female cells is dismissing criticism that he breached medical ethics with his research.

Dr. Norbert Gleicher (search) and his team at the Center for Human Reproduction in New York (search) have developed a process to transplant cells from male embryos into female embryos.

Because it is easy to differentiate male and female cells under a microscope, the technique allows researchers to track the progress of the distribution of each kind of cell within a developing embryo. The resulting hybrid embryos are destroyed after six days, when they have grown into a microscopic mixed-sex ball of a few hundred cells.

Gleicher says he hopes the research will lead to treatments for congenital diseases, like sickle-cell anemia, before they can develop.

Using embryos in scientific studies is currently legal as long as the embryos were donated for research purposes.

Critics charge Gleicher and his team breached the boundaries of medical ethics because they manipulated and destroyed healthy embryos.

Douglas Johnson, legislative director of the National Right to Life Committee (search), fears Gleicher's research is a step toward experimenting on fully developed humans.

"They took human female embryos, who were I believe normal and healthy, and then mutilated them with this research and then they killed them after six days," Johnson told Fox News. "Other researchers would do it after 14 days, and it won't stop there."

The lab created 21 hybrid embryos, of which 12 developed normally and nine abnormally. But Gleicher said that the abnormal growths were most likely due to the difficulties of thawing frozen embryos. The researchers concluded that transplants of normal embryonic cells might be a possible treatment for embryos harboring genetic defects.

Gleicher said the NRLC's concerns are unwarranted and pointed out that these embryos are donated from excess embryos created during in-vitro fertilization techniques, which never would be carried to term in the first place.

He said that although it is unknown what would happen if a hybrid embryo were transplanted into a healthy uterus, nature most likely would not allow it to implant on the uterine wall.

"There is this assumption out there that crazy stuff goes on in laboratories ... we are highly regulated," he said.

Fox News' C. Spencer Beggs contributed to the report.