The early days of how an embryo develops are shrouded in mystery, because it pulls a kind of vanishing act. Once a sperm finds an egg, it begins a roughly weeklong journey to the uterus, becoming a tiny ball of cells along the way. When it reaches its destination, it attaches to the wall of the uterus, disappearing from view.

To shed light on the process, researchers are trying to create embryo-like structures derived from stem cells, rather than sperm and eggs, so they can observe early development in the lab. These three-dimensional balls of cells could offer clues to how diseases, birth defects, and miscarriages arise, without the practical and ethical concerns raised by using actual embryos. In the latest effort, researchers in China made these structures using stem cells from macaques and tried to establish pregnancies with them in female monkeys. The experiment is described in the journal Cell Stem Cell. Although other researchers have created “synthetic” embryos before, it’s the first time anyone has done it with monkeys—animals closely related to humans—and tried to get them to implant in the uterus.

The authors started with stem cells isolated from monkey embryos that were just a few days old. Stem cells have the potential to turn into any and all body cell types and theoretically can be used to reconstitute something that resembles an embryo. After placing these cells in lab dishes, the researchers exposed them to a cocktail of nutrients and molecules to coax them into different cell types found in an embryo.

Under a microscope, the structures looked similar to blastocysts—the early stage of an embryo—at days 8 and 9 of development. They also started to form arrangements that looked like a yolk sac, which appears in early pregnancy and nourishes the embryo.

“They look very convincing,” says Kotaro Sasaki, an assistant professor of biomedical sciences at the University of Pennsylvania School of Veterinary Medicine, who studies primate embryology and human development and wasn’t involved in the study. “It looks like they have all the cell types that are present normally in embryos.”

The scientists next took some of these embryo-like balls of cells and transferred them into the wombs of eight female monkeys. In three, the structures implanted into the lining of the uterus—the first step of pregnancy. The authors confirmed the pregnancies with ultrasound and also detected the hormones progesterone and chorionic gonadotropin, which arise during pregnancy. The transplanted structures also formed early gestational sacs, fluid-filled cavities that surround a developing embryo. But the pregnancies were short-lived. These sacs disappeared after about a week. No fetuses formed.