ABSTRACT
In any discussion of critical periods it is important to distinguish between the theory and the phenomenon. The theory (Scott, 1986) deals with developmental processes in the organization of living systems on whatever level of organization the theory may be applied. The phenomenon was first established on the level of embryonic development in the classical work of Stockard (1907, 1921) who discovered in experimental studies on fish embryos that development of form could be distorted at particular periods in development and, more importantly, that the nature of the disturbance depended not on the nature of the modifying agent but on the time at which its action took place. Since that time the phenomenon has been abundantly confirmed for the embryonic developmental processes of a variety of organisms including man (Moore, 1982).
