The phrase "early embryo" refers to a very young stage of a living organism that is developing, usually in the first few weeks after fertilization. It is the earliest stage of development of a baby, plant, or animal. Full definition
Several labs have tried to create sperm in a dish from stem cells derived from early embryos, because they can differentiate into any kind of cell in the body. (sciencemag.org)
It's needed there to make hormones, including progesterone, which is essential for early embryo development. (discovermagazine.com)
But scientists have not managed to isolate such cells from farm animals, and must rely instead on injecting genes randomly into early embryos. (newscientist.com)