According to Waddington, he introduced it from psychology into embryology as means of denoting the way in which separate elements become organised into a single functioning unit. For others (e.g., George E. Coghill, 1872-1941), it referred to behaviors emerging from a total integrated pattern and progressing to increasing individuation as a consequence of inhibitory influences acting on an integrated pattern of behavior.
See Orthogenetic principle, Principle of the integration and individuation of behavior