Midline formation in insects occurs in a way that is fundamentally different way from that in vertebrates. In Drosophila a dorsal organizing region exists but it is not located on the blastopore and it does not elongate the midline. No ring-to-rod conversion occurs as in vertebrates and no moving prechordal plate exists. The midline has from the beginning the full AP extension.
On the basis of the data available at those times, a very different model has been proposed for insects (Meinhardt, 1989), which found much support by recent observations. A spot-like organizer is assumed that inhibits a stripe-forming system. The stripe forms, therefore, at the opposite ventral site, has the full length but sharpens in the course of time. This is illustrated in the following simulation.