Researchers have grown 'complete' models of human embryos using stem cells in a lab, without using sperm, eggs or a womb.
Despite giving scientists a unique insight into early embryo development, it's raising serious ethical questions.
One particular 3D model of an eight-day-old embryo may look like a normal human one, however, it was created using stem cells in an Israeli lab.
"It's an enormous achievement and will have some enormous benefits for the medical understanding of disease," Auckland University health sciences professor Andrew Shelling said.
There was no need for a womb, and rather than sperm and egg, scientists took stem cells instead.
They then added chemicals to transform them into four different types of cells.
Of the cells, 120 of them were mixed into a precise ratio, which spontaneously formed into an embryo-like structure.
It's a discovery led by the Weizmann Institute of Science in Israel.
"Under these conditions, the stem cells can make an embryo-like entity on their own," Weizmann Institute of Science lead author Jacob Hanna said.
The result isn't identical to a human embryo, but it's pretty close.
It's a move which is helping shed light on the earliest steps of human development, so that scientists can understand how diseases form and the unknown cause of miscarriages.
"It's unethical and impossible to get embryo samples in humans so we have no knowledge of what is going on there," Hanna warned.
Most countries aren't allowed to research human embryos beyond 14 days, and although embryo models fall outside those rules, they're still a massive grey area.
"What is this? Is this a human or just a group of cells? What are the implications for future use of these cells?" Prof Shelling said.
The creation of this embryo model is an ethical minefield.
It's illegal in many countries, including New Zealand, and some say the world's ethical structures are not yet ready to handle it.
"Until we've got those protections, we have to be sure these proto-embryos are not allowed to turn into babies," Massey University emeritus Professor Peter Linehman said.
"I mean this potentially could change the creation of life throughout the world."
It's a minute bundle of cells that could have massive implications.
