In Developmental psychology, a stage is a distinct phase in an individual's development. Many theories in psychology characterize development in terms of stages.
In organisms that reproduce sexually, the union of the sperm with the ovum results in a zygote, or fertilized egg, which begins a rapid series of cell divisions called cleavage, or segmentation (see mitosis).
Among the coelenterates (e.g., sponges and jellyfish), these two layers become the chief functional tissues of the adult.
In higher forms of life, a third layer of cells, the mesoderm, develops from one or both of the first two layers and fills the blastocoel, and invagination forms a digestive tract with only a single opening at this early stage.
Developmental psychologists, by and large, study the way humans develop from an embryo into a full grown adult, focusing mainly on the factors that contribute to intelligence, personality, morality, and lifestyle.
He theorized that people pass from one stage to another not just as a matter of course, but only when they are confronted with the correct type of stimulation to initiate a change.
During the formal operational stage, from age 12 to adulthood, people develop the ability to think logically and systematically and to understand abstractions and the concepts of causality and choice.
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