But at some stage during its growth, while still in the womb, the cells lose that ability because some genetic switches are flipped off or on. They become any one of the multitude of specialized body cells, and won't revert. That's convenient - it means when you cut yourself, you don't get a baby growing over the cut, you get skin. That's what Ian Wilmut and the researchers at the Roslin Institute in Scotland seem to have worked out. Then you take the DNA from the nucleus and implant it in an egg.
Differentiation is the natural process whereby cells specialize into a certain kind of cell.
As a frog embryo grows and develops, its cells differentiate into nerve cells, blood cells, fat cells and many other different kinds of cells. That's what differentiation is all about. Differentiation is important because without it an animal could never be anything but a blob of unspecialized cells. This incredible process of differentiation turns zygotes into animals and it's all controlled by the genes. An oocyte is an unfertilized egg and it has no chance of developing into an animal unless it's fertilized.