The progressive changes which occur during the life history of an individual metazoon are summed up under the term development. The adult multicellular organism differs from its early developmental stages by its size, shape, proportions, and by its parts having gradually acquired different structural and functional properties. Accordingly, several main processes involved in development, whose classification is a matter of convention, may be delimitated. Differentiation is considered one of the most important of them. In the broader sense of the word, the term differentiation is used for...