The credibility of the experiment reporting the cloning of Dolly the sheep from the cell of an adult ewe is being challenged by a leading biologist, and other scientists agree that the experiment needs to be repeated before it can accepted. The skepticism is erupting almost a year after the orginal report. The critics say that no other scientist has yet succeeded in cloning a mammal from an adult cell, although the birth of at least one calf cloned this way is said to be imminent. The cloned calves born this month were generated from fetal cells, not those of an adult cow.