FIGURE 19-6 Cell division patterns during the life cycle of Drosophila melanogaster. After fertilization, nuclei in the embryo undergo 13 rapid S phase–M phase cycles. These cycles are followed by three divisions that include a G₂ phase. All these nuclear divisions occur within a common cytoplasm and are therefore called the syncytial divisions. During late stages of embryogenesis and throughout larval development (with the exception of the nervous system), cells undergo endocycles. These cycles lead to an increase in cellular ploidy and size and hence larval growth. In the pupa, during a process called metamorphosis, imaginal disks, the tissues that give rise to the adult organs, undergo mitotic divisions and then differentiate to form adult structures. Several types of divisions are seen in the adult fly. Stem cells undergo mitotic divisions, meiosis gives rise to sperm and eggs, and endocycles create polyploid cells in the ovary. See L. A. Lee and T. L. Orr-Weaver, 2003, Ann. Rev. Genet. 37:545–578.