The research reported here is a longitudinal study designed with the purpose of discovering the physiological determinants of periods of rapid changes in maturation and development in the first year of life. René Spitz first described such changes in his book, Genetic Field Theory of Ego Formation. In analogy with the use of the term "organizer" in embryology, he called periods of rapid change "organizers of the psyche" and named as "indicators" of such changes the "smiling response" at approximately two and a half months and the "eight-month anxiety." Although Spitz's orientation was primarily psychoanalytic and his interest lay in the emotional and social implications of the affective changes in the infant, his awareness of other factors was expressed in his description of the "organizer": … a theoretical construct which designates a state of coordination and integration of a number of functions, both somatic and psychological."1