Schemes for [biological] growth
After the initial embryonic stage, many general features of the growth of different types of organisms can be viewed as consequences of the nature of the elements that make the organisms rigid. In plants, as we have discussed, essentially all cells have rigid cellulose walls. In vertebrate animals, rigidity comes mainly from bones that are internal to the organism. In arthropods and some other invertebrates, an exoskeleton is typically the main source of rigidity. Growth in such organisms usually then proceeds by adding soft tissue on the inside, then periodically moulting the exoskeleton. In a first approximation, the mechanical pressure of internal tissue will typically make the shape of the exoskeleton form an approximate minimal surface.