Abnormalities in somite segmentation following heat shock to Xenopus embryos.

The typical abnormality induced by a 15 min shock at 37degreesC is a single discrete length along the somite file within which segmental boundaries are absent or irregular. The two sides of the same embryo present a similar but not necessarily identical appearance. Usually all the embryos in a treated batch show abnormalities of similar severity. Survival of treated embryos, the details of the visible malformations, and temporal aspects of the phenomenon have been studied. The results indicate a temperature sensitive period that traverses the neurula, from head to tail at about the same rate as the somites form, but some hours beforehand. The temperature sensitive process is not associated with cell determination and differentiation, and there are reasons for thinking that the specification of the normal somite number occurs independently. The results are discussed in relation to Cooke & Zeeman's model of a wave front interacting with an oscillator.