Fig. 13.

Summary of the morphogenetic movements of the lateral mesoderm and ectoderm at the head–trunk interface, projected onto the avian embryo. During the growth of the chick embryo between HH10 (E1$\frac{1}{2}$) and HH20 (E3), the trunk lateral mesoderm and ectoderm becomes rostrocaudally compressed (A, red arrows) but mediolaterally expanded (B, black arrows). Eventually, tissues originating from a position next to the most rostral somites move around the pharyngeal arches and along the floor of the pharynx towards the mandibular arch. The morphogenetic movements may be driven by relative growth processes, by space constraints executed by increasing cranial flexure and the swelling pharyngeal arches, by convergence extension movements or by concerted active migration of lateral mesodermal and ectodermal cells. Importantly, this cell movement begins before the release of neural crest cells and HMP, and serves as a conveyor belt for non-migratory cells, aiding the assembly of the hypopharyngeal apparatus.