The subcommissural organ.
D000818:Animals; D001001:Anura; D002415:Cats; D004285:Dogs; D004622:Embryo, Mammalian; D004625:Embryo, Nonmammalian; D004805:Ependyma / Q000033:anatomy & histology*; D006801:Humans; D008636:Mesencephalon / Q000033:anatomy & histology* / Q000378:metabolism; D008854:Microscopy, Electron; D009489:Neurosecretion