In embryos of Drosophila melanogaster all the nuclei in the syncytial egg divide with global synchrony during the first 13 mitotic cycles. But with cellularization in the 14th cycle, global mitotic synchrony ceases. Starting about one hour into the 14th interphase, at least 25 ‘mitotic domains’, which are clusters of cells united by locally synchronous mitosis, partition the embryo blastoderm surface into a complex fine-scale pattern. These mitotic domains, which are constant from one embryo to the next, fire in the same temporal sequence in every embryo. Some domains consist of a single cell cluster straddling the ventral or dorsal midline. Most consist of two separate cell clusters that occupy mirror-image positions on the bilaterally symmetric embryo. Others comprise a series of members present not only as bilateral pairs but also as metameric repeats. Thus a domain can consist of either one, two, or many (if metamerically reiterated) clusters of contiguous cells. Within each cluster, mitosis starts in a single cell or in a small number of interior cells then spreads wave-like, in all directions, until it stops at the domain boundary. Each domain occupies a specific position along the anteroposterior axis—as determined by the expression pattern of the engrailed protein, and along the dorsoventral axis—as determined by cell count from the ventral midline. The primordia of certain larval structures appear to consist solely of the cells of one specific mitotic domain. Moreover, cells in at least some mitotic domains share specific morphogenetic traits, distinct from those of cells in adjacent domains. These traits include cell shape, spindle orientation, and participation by all the cells of a domain in an invagination. The specialized behaviors of the various mitotic domains transform the monolayer cell sheet of the blastoderm into the multilayered gastrula. I conclude that the fine-scale partitioning of the newly cellularized embryo into mitotic domains is an early manifestation of the commitment of cells to specific developmental fates.

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