Gastrulation, the process where an embryo reorganizes itself from a hollow sphere into a multilayered structure, is considered a “black box” of human development. This is because human embryos are typically not cultured for longer than 14 days because of bioethical concerns, and gastrulation occurs between 17- and 21-days post-fertilization.
Click here for original story, Researchers generate human embryo-like structures that include extraembryonic tissue
Source: Phys.org