This chapter discusses the imaginal discs of Drosophila and shows that both heritable and nonheritable cellular decisions are made in these systems. Although specification itself is nonheritable and may change during cell division, there are rigid constraints on the possible changes of specification that can occur. The chapter discusses these constraints as far as they are understood. Imaginal discs undergo the differentiation at any time, by transplanting them into the host larvae that are about to undergo metamorphosis. The implanted disc responds to the hormonal conditions of the host and metamorphoses with it. The mature imaginal disc is not merely a disc, it is a layer of cells with a specific morphology and regular folding pattern. The pattern of folds, specific patterns of cell death and regional differences in mitotic rate, indicate that important differences already exist between the cells in different parts of a disc. The chapter explains certain restrictions on the degree and direction of the respecification that take place during cell division. These restrictions result in the gradients of developmental capacity in imaginal discs that account for the occurrence of regeneration and duplication from complementary imaginal disc fragments. Specifications are associated with a fine tuning mechanism, whereby a specified cell inhibits the nearby cells from attaining identical specification. The nature of the molecular events leading to the establishment and maintenance of both determined and specified states in cells remains to be elucidated. © 1974, Academic Press Inc.
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