Scientists at Harvard have identified a previously unknown embryonic signal, dubbed Toddler, that instructs cells to move and reorganize

"Whether human or animal, vertebrate or invertebrate, nearly every creature begins life as a tiny clump of cells. Before those cells can begin blossoming toward being a fully formed organism, however, they first must reorganize themselves into layers, each of which goes on to form complex structures such as internal organs, skin, muscle, and bone..."
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