The presumptive somite mesoderm of the primitive streak stage does not form somites if it is totally isolated from other tissues. Hence it has been proposed that the somites form by embryonic induction, but since no single tissue has been found to be essential, this concept has become less credible. Instead, the segmentation process appears to take place in two major steps: (1) as the node regresses, an early programming establishes the number and bilaterality of the somites, occurring as a wave of change passing from the anterior to the posterior end of the body; (2) after an interval of about 20 h, a second wave passes down the body and converts the programmed cells into somites. This theory is supported by morphological and experimental evidence.