in developmental biology, two philosophically distinct models have dominated the field of pattern formation. The subject matter of the field is, broadly speaking, the question of how spatial patterns arise during the course of transition of an egg into an adult. The patterns could be those formed by the stripes along a zebra's back or sensory hairs (bristles) on the thorax of insects. The two models ascribe different types of ultimate causes responsible for the final pattern.

One model goes by the name of