abnormal axial mesoderm increased axial mesoderm size csmith MPheno.ontology expanded axial mesoderm 2013-11-14T11:49:43Z greater than average size of the part of the middle of the three primary germ layers of the embryo which includes the prechordal mesoderm and the chordamesoderm and gives rise to the prechordal plate and to the notochord MP:0012274