The cellular architecture of the blastoderm is that of a simple monolayer. Therefore, embryonic development can, in short, be described as the process of transforming this two-dimensional structure into a three-dimensional structure in which the basic body pattern (Sander 1976, 1983) can already be recognized. This topological transformation is achieved by means of morphogenetic movements. Morphogenetic movements may take place either (i) by growth and infolding of certain regions of the monolayer, without alteration of its original arrangement as, for example, occurs in the epidermal anlage; or (ii) by segregation of individual cells or coherent cell arrays from the monolayer into the interior of the embryo, which thereby lose their original contacts with the other regions; examples include the segregation of the neuroblasts and the invagination of the ventral furrow or the midgut. Indeed, most of the elementary morphogenetic movements found in embryos of other animal species, e.g. invagination, involution, ingression, delamination etc., also occur during embryogenesis in Drosophila.
KeywordsGerm Layer Mesodermal Cell Germ Band Ventral Plate Morphogenetic Movement
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