It's being called "monstrous" by opponents, but is hailed by the scientific community. Newcastle University in England has created the first ever human-cow embryos.
A team grew the hybrid embryos by putting human DNA into eggs taken from cows' ovaries, which had most of their genetic material removed.
The embryos survived for three days and are intended to provide a limitless supply of stem cells to develop therapies for diseases such as Alzheimer's, Parkinson's and spinal cord injuries.
"Cells grown using animal eggs cannot be used to treat patients on safety grounds ," John Burn, head of the Institute of Human Genetics at Newcastle University, said in a statement. "But they will help bring nearer the day when new stem cell therapies are available."