mesoderm
Mes"o*derm, n. Etym: [Meso- + Gr. (Biol.) (a) The layer of the blastoderm, between the ectoderm and endoderm; mesoblast. See Illust. of Blastoderm and Ectoderm. (b) The middle body layer in some invertebrates. (c) The middle layer of tissue in some vegetable structures.