embryonic structure embryonic corpus cardiacum The corpora cardiaca precursor first appear during stage 10 as two pairs of head mesoderm cells between the roof of the stomodeum and the inner surface of the brain primordium. Between stages 11 and 15, these cells migrate posteriorly, gradually increasing in number. During stage 15, they fuse with the precursors of the corpus allatum from the gnathal mesoderm to form the ring gland. James Malone P1 CC corpus cardiacum primordium FBbt:00001720 The corpora cardiaca precursor first appear during stage 10 as two pairs of head mesoderm cells between the roof of the stomodeum and the inner surface of the brain primordium. Between stages 11 and 15, these cells migrate posteriorly, gradually increasing in number. During stage 15, they fuse with the precursors of the corpus allatum from the gnathal mesoderm to form the ring gland. corpus cardiacum placode