part of develops into ectoderm embryonic thoracic segment dorsal thoracic disc primordium ventral thoracic disc primordium developing embryonic structure Keilin organ primordium thoracic limb primordium 2019-08-23T09:38:06Z fly_anatomy.ontology FBbt:00048408 Cluster of cells in the ventrolateral ectoderm in the embryonic thorax that will eventually give rise to a dorsal imaginal disc, a ventral (leg) imaginal disc and a Keilin organ in the larva. The three different cellular identities cannot be distinguished at stage 11, but can by stage 14. There is one per thoracic hemisegment (six total).