The formation of somites in Xenopus.

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. In a pre-hatching Xenopus larva of stage 28 (Nieuwkoop & Faber, 1956) it can be observed that the myotomal cells are already elongated antero-posteriorly and that each cell stretches the length of the entire somite. This is even true in the most newly formed somites as can be seen in Fig. 1B and Fig. 2C. The cells of the dermis do not seem to pass through a segmental phase and no somite contains a myocoel. It thus appears that somite formation in Xenopus does not follow the typical vertebrate pattern. A fuller description of this process in Xenopus now follows.