But in the future, they may develop into far more complex forms, the researchers said, such as a beating human heart connected to a rudimentary brain, all created from stem cells. Such a Sheef might reveal important clues about how nerves control heartbeats. Scientists might be able to use other Sheefs to test out drugs for diseases such as cancer or diabetes.

Whatever else, it is sure to unnerve most of us.

Established guidelines for human embryo research are useless for deciding which Sheefs will be acceptable and which not, Dr. Aach argued. Before scientists get too deeply into making Sheefs, some rules must be put in place.

Dr. Aach and his colleagues urged that certain features be kept off limits: Scientists, for example, should never create a Sheef that feels pain.

“We’re going to have to get a lot of input from a lot of quarters,” Dr. Aach said in an interview. “The problems are just too big.”

Scientists began grappling with the ethics of lab-raised embryos more than four decades ago.

In 1970, the physiologist Robert G. Edwards and his colleagues at the University of Cambridge announced they had been able to fertilize human eggs with sperm and keep them alive for two days in a petri dish. During that time, the embryos each divided into 16 cells.