ABSTRACT
Whether the notochord arises from endoderm, mesoderm or ectoderm has been debated almost since von Baer identified the notochord (chorda dorsalis) as the precursor of the vertebral column in 1828. One hundred years later Goodrich (1930) concluded that the notochord always arises from the dorsal roof of the endodermal archenteron. Since then, it has been determined that endoderm and mesoderm may both exist in the roof of the archenteron and that the origin of the notochord varies among different groups of vertebrates. Part of the difficulty in assessing the origin of the notochord is that (i) the mesoderm is a secondary germ layer (indeed, often the literature refers to an endomesoderm), and (ii) that anterior and posterior regions of the notochord in vertebrates can and do arise from different populations of cells. Similarities between notochord and cartilage encourage us to conclude that the notochord is mesodermal. So too does the formation, during early development, of what is termed the chordamesoderm as notochord precursors segregate from the remaining mesoderm under the influence of the dorsal organizer (Hensen’s node).
In the next chapter, we turn to the various functions of the notochord in early development. The number of functions may surprise you. They certainly explain why the notochord has been conserved throughout chordate evolution.
