The formation of somites and early myotomal myogenesis in Xenopus laevis, Bombina variegata and Pelobates fuscus.

Myogenesis in Xenopus laevis and in Bombina variegata is similar despite differences in the structure of the nonsegmented mesoderm and in the formation of the myotomes. In X. laevis the nonsegmented mesoderm consists of two cell layers with the premyocoel between them. During somitogenesis the premyoblasts rotate covering subsequently the whole myotome length. In B. variegata the premyocoel is absent. The myotomal cells change their shape and elongate, attaining ultimately the whole myotome length. The morphologically mature mononuclear muscle cells in both species result from myogenesis beginning in similarly arranged myoblasts. The multinuclear myotubes arise in the swimming tadpole (stage 45). The structure of the nonsegmented mesoderm and of the newly formed myotomes in Pelobates fuscus is similar to that of B. variegata, while the process of myogenesis is different. It begins in the multinuclear myotubes. The stage of morphologically mature mononuclear muscle cells was not observed in the light microscope. The results suggest that myotomal myogenesis is related neither to any particular type of nonsegmented mesoderm structure nor to any specific mode of myotome formation.