The blood leaves the heart through the truncus arteriosus, which gives off six pairs of aortic arches. The aortic arches form angiogenetic clusters in adult fish. The ductus arteriosus in mammals is a vestigal part of the sixth aortic arch on the left side. Morphogenetic Movements a. Delamination is the splitting of one cell sheet into two parallel sheets. Epiboly is the spreading movement of an epithelial sheet over a deeper cell layer.
Invagination is the local inward buckling of an epithelial sheet.
Involution is the migration of single cells from an epithelial layer into an embryonic cavity. Phylotypic Stage a. Among adult vertebrates, mammals remain closest to the phylotypic organization. The secondary jaw joint of mammals is part of the phylotypic organization of vertebrates. Multiple aortic arches are normal in fish but occur only sporadically as atavisms in mammals.