Definition: "Blastomere which lies adjacent to the prospective dorsal midline, anterior to its sister cell, blastomere D2.1.1, in the third tier of blastomeres. It contributes to mostly endodermal and mesodermal structures including the heart, head mesenchyme, notochord, somites, lateral plate mesoderm, foregut, hindgut, and a few cells of ectodermal origin (in cranial ganglia and neural crest)."