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| Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Forming Complex Pattern: Utilizing Positional Information to Establish Cell Fates Figure 16-20. Cellularization of the Drosophila embryo. The embryo begins as a syncytium; cellularization is not completed until there are about 6000 nuclei. (a) and (b) Scanning electron micrographs of embryos removed from the eggshell. (a) A syncytium-stage embryo, fractured to reveal the common cytoplasm toward the periphery and the central yolk-filled region. The bumps on the outside of the embryo are the beginning of cellularization, in which the plasma membrane of the egg moves from the outside inward. (b) A cellular blastoderm embryo, fractured to reveal the columnar cells that have formed by cell membranes being drawn down between the elongated nuclei to create some 6000 mononucleate somatic cells. (c) Schematic diagram of the changes taking place during cellularization. (Modified from F. R. Turner and A. P. Mahowald, Developmental Biology 50, 1976, 95.)

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