abnormal somite development abnormal somite segmentation clock 2014-01-05T23:31:11Z MP:0012669 MPheno.ontology anomaly or loss of the oscillatory mechanism governed by periodic shifts in gene expression and operating in the presomitic mesoderm that drives the regular periodicity with which somites are formed in the course of vertebrate segmentation. The total number of somites formed and the periodicity with which they are produced are species-specific parameters (e.g. humans have 33 somites with a somitogenesis period of approximately 6 hours while mice have 65 somites with a period of approximately 2 hours) csmith