These embryo-like structures are at the very earliest stages of human development: They don’t have a beating heart or a brain, for example. But scientists say they could one day help advance the understanding of genetic diseases or the causes of miscarriages.

The research raises critical legal and ethical questions, and many countries, including the US, don’t have laws governing the creation or treatment of synthetic embryos.

The pace of discoveries in this field and the growing sophistication of these models have alarmed bioethics experts as they push ever closer to the edge of life.

Researchers hope these model embryos will shed light on the “black box” of human development, the period following 14 days after fertilization, which is the agreed limit for scientists to grow and study embryos in a lab.

Right now, the synthetic model human embryos are confined to test tubes. It would be illegal to implant one in a womb, and animal research with stem cells from mice and monkeys has shown that even when scientists have attempted to implant them, they don’t survive – probably because researchers haven’t figured out how to fully replicate the conditions of pregnancy.

According to the CNN