The origin of the cells which form the aortic arches in the chick embryo was determined by tracing the movements of tritiated thymidine-labeled grafts excised from medium streak (MS) to head process (HP) stage embryos and transplanted to the epiblast, streak and endoderm-mesoderm layers of similarly staged recipient embryos.
Between the MS and HP stages, the preaortic arch cells migrate in epiblast to the primitive streak, invaginate, and move anteriorly and laterally from the streak into mesoderm; at the HP stage they are lateral to the cells which will form the anterior parts of the dorsal aortae, and medial to the heart-forming regions. The most anterior prearch cells form the first arch and the more posterior cells form the more posterior arches. The prearch cells are closely associated during migration with the endoderm cells which will form the adjacent branchial arches and clefts.
Cells which will form the posterior end of the aorta are still in the epiblast layer at the late medium streak (LMS) stage, but have moved to the primitive streak at the HP stage; they migrate from the streak into mesoderm posterior and medial to the mesoderm destined for the heart and aortic arches.