After cellularization, mitotic divisions are no longer synchronous, due to position specific differences in the length of interphase 14. Gastrulation in Drosophila is superficially different from that of vertebrates, but does begin with a hollow spheroid of cells and end with three germ layers. The first major movements of gastrulation occur along a ventral furrow , where the presumptive mesoderm moves inside. Somewhat later, the neural tube is formed ventrally, from neurogenic ectodermal cells on either side of the midline and procephalic neuronal ectoderm in the head. During germ band elongation , the ventral cells move toward the posterior and up over the back.
Segmentation occurs when the germ band is elongated.
After germ band shortening the dorsal opening closes and the head structures move inside the anterior. The imaginal disc cells that will form the adult are set aside from the remaining ectoderm. Additional organogenesis occurs during the latter half of embryogenesis. Larval development, metamorphosis, and adult morphology . The embryo hatches as a first instar larva or imago, in which each segment is marked by characteristic rows of bristles on the ventral side and hairs on the dorsal side.