The pre-somitic mesoderm commits to the somitic fate before mesoderm becomes capable of forming somites. Vertebrates like the zebrafish use oscillating gene expression to define segments known as somites. On the flagelliform and ventral portion of the abdominal somites, they measure . The muscles of the tongue evolved in amphibians from occipital somites. The thorax is divided into two groups of somites. The next four somites, 3 to 6, all bear pairs of spiracles. In the chick embryo somites are formed every 90 minutes. The abdomen is composed of five somites, all of which lack appendages except the last. Spots on the thoracic somites are coalescing. The somites appear on both sides of the neural tube simultaneously. Chicken embryos have 50 somites; mice have 65, while snakes have 500. In current crustaceans, several of those somites may be fused. They can aid in inducing the lateral characteristics in somites. Cells at this point will mature and form a pair of somites. The somitomeres organize into somites which grow in pairs.