Precursors of bones can be identified quite early in the growth of most vertebrate embryos. Typically the cells involved are cartilage, with bone subsequently forming around them. In hardened bones growth normally occurs by replication of cartilage cells in plates perhaps a millimeter thick, with bone forming by a somewhat complicated process involving continual dissolving and redeposition of already hardened material. The rate at which bone grows depends on the pressure exerted on it, and presumably this allows feedback that for example prevents coiling. Quite how the complicated collection of tens of bones that make up a typical skull manage to grow so as to stay connected—often with highly corrugated suture lines—remains fairly mysterious.