What does the term 'chimera' conjure up for you? The fire-breathing monster of Greek mythology with the head of a lion, body of a goat and tail of a serpent? Or the biological version of a chimera – a mixed organism such as a cultivated plant consisting of two genetically different kinds of tissue as a result of mutation? British scientists are challenging the Human Fertilisation & Embryology Act 1990, which makes it illegal to mix human and animal eggs and sperm. They have submitted applications to the Human Fertilisation and Embryology Authority to proceed with experiments which will involve fusing human cells with eggs from rabbits, cows and goats. They hope to benefit from an established exception, the 'hamster test', by which doctors are allowed to test the quality of human sperm by fertilising a hamster egg and then destroying it no later than at the two-cell stage.

The scientists' aim is to create 'chimeric' embryos that would be 99.9% human and 0.1% animal in order to produce embryonic stem cells. Stem cells are known to be basic 'building-blocks' of the body and can develop into all other types of cells. If permission is granted, the embryos would be destroyed after fourteen days. At present human eggs are supplied by IVF patients, but as demand increases these are no longer sufficient. If given the go-ahead, Dr Stephen Minger of King's College London hopes to use cloned hybrid embryos to create stem cells carrying the defects responsible for conditions such as Parkinson's Disease and Alzheimer's, thus facilitating research.

Opponents have expressed strong views; Josephine Quintavelle of Comment on Reproductive Ethics said: "This is abhorrent ... there is a basic human feeling that animals and humans do not mix in these areas." Calum Mackellar of the Scottish Council on Human Bioethics said "In this kind of procedure you are mixing at a very intimate level animal eggs and human chromosomes and you may begin to undermine the whole distinction between animals and humans."