Is it beating heart muscles, or the first signs of a spinal column? Perhaps it is the emergence of brain activity, or the earliest hints of tiny limbs?

Members of the public are being asked by scientists to tell them how far artificial human embryos should be allowed to develop in the lab before research starts to feel too “Frankenstein”.

In a new code published today, scientists agree that human embryo “models” made using stem cells should never be implanted into human or animal wombs or grown in a lab in a way that would allow them to develop into full foetuses and “viable” living beings.

It has also been agreed that these artificial embryo models “shall only be cultured in vitro [outside the body] for the minimum time needed to achieve the scientific objectives proposed”, with a clear plan in place for terminating their development. Researchers will also need to consider the consent and wishes of the stem cell donors.

Under existing legislation, scientists can only develop an embryo for 14 days in the laboratory before they have to terminate their studies and destroy it.

Beyond 14 days, the embryo starts to take on a more recognisably human structures. Researchers can, however, create embryo “models” by taking stem cells and encouraging them to assemble 3D structures — a “bubble of cells” that mimics the features of early-stage embryos and could boost the success rate of IVF treatment and reduce miscarriages, while also providing a safer way to test new drugs or research how to grow organs for transplant.

Researchers from the Francis Crick Institute have set up an exhibit at the Royal Society’s Summer Science exhibition in London and are asking members of the public to tell them where the “limits” should be on how far these embryo models should be allowed to develop.

From the public engagement so far “the most common reaction was fascination rather than ‘Frankenstein’,” said Professor Emily Jackson, a law professor at the London School of Economics.