part of develops into developing embryonic structure primordium pars intercerebralis primordium Primordium of the late embryonic pars intercerebralis. At stage 12, the primordium of each hemisphere forms a narrow cluster of approximately 40-50 cells, in the antero-medial procephalon, behind the furrow that separates the procephalon from the clypeolabrum. The primordium then invaginates from the procephalic ectoderm between stages 11 to 13. During stages 13 to 14, the invagination pinches off the surface ectoderm and forms small vesicles in between the outer epithelium and the brain surface. fly_anatomy.ontology FBbt:00005511 central brain primordium embryonic pars intercerebralis anterior ectoderm derivative