The Pattern of Embryonic Cell Divisions

Abstract

Two different periods of mitotic activity can be distinguished during the embryonic development of Drosophila melanogaster. The first mitotic period takes place prior to cell formation, and during this period the zygotic nuclei go through 13 rounds of mitoses (Zalokar and Erk 1977, Foe and Alberts 1983). The first seven zygotic divisions are synchronous, leading to a syncytium of 128 nuclei distributed as an ellipsoid centrally within the yolk (Rabinowitz 1941 a). During the course of the next three mitotic cycles (8–10), most of the nuclei approach the egg surface stepwise to form the syncytial blastoderm, whereas about 200 nuclei (the presumptive vitellophages) remain central (Rabinowitz 1941 b), and another 17–18 (Zalokar and Erk 1977) are incorporated by the posterior pole plasma to form the pole cells. Three further, parasynchronous mitoses (Foe and Alberts 1983) bring the number of syncytial blastoderm nuclei to approx. 5000 (Zalokar and Erk 1977, Turner and Mahowald 1977), terminating the first period of mitotic divisions. The second period of mitotic activity affects the embryonic cells after the blastoderm stage. Embryonic cells do not divide very frequently. Up to the end of embryogenesis, and with the exception of the precursors of the CNS and the epidermal sensilla, and of the pole cells, most cells of the Drosophila embryo have to undergo merely two or three divisions.