Anterior–posterior differences in vertebrate segments: specification of trunk and tail somites in the zebrafish blastula

During vertebrate embryogenesis, the primary body axis grows posteriorly and is concomitantly segmented into somites, the precursors of the vertebral column, skeletal muscle, and dermis. The somites arise sequentially, with the anterior somites that give rise to the cervical vertebrae created early. The more posterior somites that become the thoracic, lumbar, and sacral vertebrae form at progressively later times. During the axis elongation period, the embryo must parse the somite precursors ap