The Collaborative International Dictionary of English v.0.48:

Mesoderm \Mes"o*derm\, n. [Meso- + Gr. de`rma skin.] (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.
       [1913 Webster]

WordNet (r) 3.0 (2006):

mesoderm
    n 1: the middle germ layer that develops into muscle and bone
         and cartilage and blood and connective tissue [syn:
         mesoderm, mesoblast]