ABSTRACT
The cell movements and contacts in somite formation in Xenopus are unusual. The vertebrate somite as seen in section is traditionally described as a rosette and is formed by pinching off a group of cells from the paraxial mesoderm; these cells then increase intercellular contacts. Typical rosettes with cells radiating from the myocoel may be seen in the Axolotl (Fig. 1 A). Not all the cells of a rosette have the same fate. The lateral ones become dermis and spread out under the epidermis forming a continuous sheet with their counterparts from neighbouring somites. The majority of the somitic cells, which are myotomal, will elongate antero-posteriorly and fuse end to end with other cells of the same somite (Przybylski & Blumberg, 1966; Riabova, 1966). In this way myotubes are formed which stretch from one end of the somite to the other and obliterate the myocoel. Somites are formed in an orderly anterior-posterior succession.