Cephalic axial muscles of craniates and their innervations | 1:a upplagan

The phylum Myomerozoa (Bjerring 1984) unites two groups of animals, to wit, Craniata and Myochordata. Regardless of differences, all studied myomerozoan embryos, rounded or flattened, present a middle germ layer ) composed of a median band of cells, the notohylic mesoderm, and two lateral masses of cells, the pleurohylic mesoderm (Bjerring 1967, 1968). In the myochordates (the lancelets) the notohylic mesoderm throughout its entire length gives origin to notochordal tissue which during later development grows forward; accordingly the adult lancelet notochord continues a good distance in front of the neuronal tube ). In the craniates, however, the notochord never extends beyond the hypophysis because notochordal formation is deleted in the most anterior part of the notohylic mesoderm. When isolated under cultural condition outside the craniate embryo this anterior, or emprosthial, part of the notohylic mesoderm nevertheless may give origin to notochordal tissue (Holfreter 1938; Takaya 1953).