As soon as cellularization is complete, gastrulation begins. As one piece of evidence, haploid embryos go through a 15th round of nuclear division prior to cellularization. Initiation of bulk RNA synthesis results from the lengthening of the interphase period and will occur earlier if the nuclear cycle is arrested by drug treatment. Somatic cells begin to take on position specific differences in behavior almost as soon as they are formed at cellular blastoderm. For gastrulation to occur correctly, cells at specific positions must go through different, stereotyped changes in shape and behavior.
Mitotic divisions are synchronous during the cleavage and syncytial blastoderm stages.
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.