During embryonic development, many animals form a number of temporary structures along their anterior-posterior axis-somites. Cells in somites differentiate into sclerotome, which gives rise to vertebral column, dermatome, which gives rise to dermis, and myotome, which gives rise to skeletal muscle. During the past three decades, several models have been proposed to explain the formation of somites, including the clock and wavefront model, the react ion-diffusion type model, the clock and induction model, the clock and trail model, and others. These models, albeit not perfect, serve to explain different aspects of somitogenesis. Most of these models propose the concept of segmental clock. Studies on expression patterns of c-hairy 1 and c-hairy 2 in chick, I-fng in chick and mouse, and her 1 and Delta C in zebrafish have provided evidence for the existence of segmental clock.