As a consequence of generating and reshaping the epithelia of their ectodermal and endodermal germ layers and (for those species that have them) the mesenchymes that make up the mesodermal germ layer and neural crest, embryos take on one of approximately three dozen stereotypical “body plans.” These body plans can be asymmetric (sponges), radially symmetric (e.g., hydra and sea urchins), bilaterally symmetric (e.g., planaria, insects), or “bilaterally asymmetric” (vertebrates; see the last section of Chapter 7). While it is obvious that nothing in development, or in any other domain of biology for that matter, can occur without the participation of physical mechanisms, the high degree of structural and dynamical complexity of most living systems makes it exceedingly difficult, in general, to follow the workings of basic physical principles or appreciate their roles in generating characteristic biological phenomena. It is therefore remarkable to how great an extent (as the preceding chapters have shown) the tissue generation and reshaping processes leading to embryonic body plans can be accounted for by physical forces and properties – adhesion, viscoelasticity, dynamical multistability, network formation, positive and negative feedback dynamics, diffusion – that also govern the behaviors of nonliving condensed materials, that is, by “generic” physical mechanisms.

The ability to understand aspects of early development in generic physical terms can be attributed, in part, to the likelihood that the original multicellular organisms were simple, loosely organized, cell masses, whose forms were determined to a great extent by their inherent physical properties (see Chapter 10).