While germ band extention is accompanied by cellular interdigitation, germ band retraction is accompanied by the transition from a parasegmental to segmental division of the embryo. In the process of segmentation, deep ventral-lateral grooves form, corresponding to the incipient segmental boundaries. The interior aspects of these segmental boundaries are the sites for future muscle attachment. Genes involved in germ band retraction include the Epidermal growth factor receptor , pebbled and serpent . During segmentation , the segregation of imaginal discs takes place.
Imaginal discs are sacs of cells that give rise to adult structures during pupal metamorphosis.
Gap genes structure the striped pattern of pair rule genes. The striped pattern leads to the segmental array of segment polarity gene transcription. Here is a developmental hierarchy, each chain in the link of events made possible by what has preceded it, and making possible what has yet to come. It takes about two hours, beginning 11 hours after the start of development. Dorsal closure is the process whereby the stretched amnioserosa is covered by epidermal cells that will ultimately fuse at the dorsal midline.