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https://www.nature.com/articles/d41586-022-02334-2
That period (8.5 days) is long enough for the brain regions to develop, the heart to start beating, and the neural and gut tubes to form. These synthetic embryos look a lot like natural embryos that form when mouse sperm meets egg, but they “were not 100% identical”, Hanna says. “You can see some defects and some changes in the organ size.”