Lit from below by the microscope’s soft glow, the translucent mouse embryos looked exactly as they should. On day 3 they began to elongate, from spheres into cylinders. On one end, the neural tube started to fold around day 6, on the other a tail began to bud. By day 8, a beating heart began to circulate blood through vessels forming around the embryo’s yolk sac.

But these embryos weren’t the product of an egg and a sperm. They weren’t even growing in the uterus of a female mouse. They were developed inside a bioreactor, and made up entirely of stem cells cultured in a Petri dish.

The experiments, performed at the Weizmann Institute of Science in Israel and published Monday in Cell, mark the first time researchers have grown fully synthetic mouse embryos — that is without the use of sperm or eggs — outside the womb.