Segmentation

Snakes have hundreds of vertebrae, while mice and humans have only a few dozen. This difference is explained by the “clock and wavefront” model. Every time an embryo’s “clock” ticks, a “wave” of activity creates a segment, working its way from head to tail. Some animals’ clocks tick fast, and others slow. Scientists are working to better understand what makes up these “clocks” and what sets their pace.

Illustration for Knowable magazine's Special Report: Building Bodies