part of overlaps presumptive embryonic/larval nervous system developing embryonic structure primordium midline primordium FBbt:00005528 embryonic mature midline Region of the developing embryonic central nervous system that is derived from the mesectoderm and found at the midline (Kearney et al., 2004). The mesectodermal cells have undergone further cell division and differentiation and a metameric organization is now more apparent (Kearney et al., 2004). fly_anatomy.ontology Kearney et al. (2004) refer to this as 'mature midline' from stage 13, despite the presence of immature cells. mesectodermal derivative midline