part_of part of part of develops from immediate transformation of membrane organ meninx https://upload.wikimedia.org/wikipedia/commons/4/48/Illu_meninges.jpg meningeal Whereas cyclostomes and fishes only have a single envelope called the primitive meninx, amphibians have two layers, consisting of an outer dura mater which is extremely dense and protective, and a pia-arachnoid or secondary meninx which is more delicate and vascular. Mammals have three meninges: pia mater (which follows all the convolutions of the brain and is the most interior), the arachnoid layer (which is delicate and sends strands to the pia mater), and the dura mater (the outer, more protective meninx). ectoderm-derived structure future meninx meningeal cluster