 During development in both vertebrates and invertebrates, cell lineage restriction boundaries and signaling centers are formed in order to ensure proper differentiation of the body.Chemical signals, like "shh" from the ZLI, are often released from these boundaries and compartments in a concentration gradient (the chemicals are in much higher concentrations closer to the source) and confer identity to flanking regions.Other genes differentially expressed in these flanking regions aid in ensuring proper differentiation (see Signaling).