1. Stowers Institute for Medical Research, Kansas City, Missouri 64110, USA
2. Vertebrate Development Laboratory, Cancer Research UK, London Research Institute, London WC2A 3PX, UK
3. Howard Hughes Medical Institute, Kansas City, Missouri 64110, USA

Correspondence to: Olivier Pourquié^1,3 Correspondence and requests for materials should be addressed to O.P. (Email: olp@stowers-institute.org).

The vertebrate body axis is subdivided into repeated segments, best exemplified by the vertebrae that derive from embryonic somites. The number of somites is precisely defined for any given species but varies widely from one species to another. To determine the mechanism controlling somite number, we have compared somitogenesis in zebrafish, chicken, mouse and corn snake embryos. Here we present evidence that in all of these species a similar 'clock-and-wavefront'^{1, 2, 3} mechanism operates to control somitogenesis; in all of them, somitogenesis is brought to an end through a process in which the presomitic mesoderm, having first increased in size, gradually shrinks until it is exhausted, terminating somite formation. In snake embryos, however, the segmentation clock rate is much faster relative to developmental rate than in other amniotes, leading to a greatly increased number of smaller-sized somites.

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