Figure 1.

The formation of lamina (LA) and medulla (ME) neuropils by duplication from a single proliferation center, or neuroblast (Nb) anlage. Cells of the LA and ME cortices proliferate in antiparallel directions from this center, their axons growing in temporal sequence between the two (see: e.g., Figure 3 in Meinertzhagen, 1973). A crossed fiber tract, the external chiasma, necessarily forms between the two as the coordinated outcome of two events, cell proliferation into LA and ME cortices in antiparallel directions, and the directed growth of axons so as to penetrate the LA but grow across the face of the ME. The retinotopic sequence of innervation of the LA matches the order of projection of ME cells to the lobula (LO).