see Growth curve
Tissue growth is initiated by external factors affecting a more or less complicated system and leading to chains of reaction which may be identical, or at least similar, even in cases in which the primary causes differ. These primary causes are called “formative stimuli” in order to distinguish them from functional stimuli. It is probable that essentially both kinds of stimuli are identical and act in a similar manner. The difference in the result of both kinds of stimulation depends probably on the quantitative differences in strength and time, and on the differences in the systems on which they act.