Embryonic development is a genomically encoded construction process in which cells acquire their identities and build organs within a three-dimensional embryonic environment. A central question in developmental biology is: once cells know who they are, how do they construct the organs they are set to form? We address this question by studying the relatively simple system of sea urchin skeletogenesis, focusing on the interplay between gene regulatory networks (GRNs) that specify cell identity and cytoskeletal and adhesion protein networks that drive cell behavior and morphogenesis.