ventral midline
In protostomes (such as insects, snails and worms) as well as deuterostomes (vertebrates), the midline is an embryonic region that functions in patterning of the adjacent nervous tissue. The ventral midline in insects is a cell population extending along the ventral surface of the embryo and is the region from which cells detach to form the ventrally located nerve cords. In vertebrates, the midline is originally located dorsally. During development, it folds inwards and becomes the ventral part of the dorsally located neural tube and is then called the ventral midline, or floor plate.