Beginning at mid-neurulation, a wave of somite segmentation passes down the axolotl body axis in a cephalocaudal direction. At 20 degrees C a somite forms every 2.57 hr. Fate-mapping of the presomitic mesoderm indicates that the primordia for the next few somites occupy nearly the same space that they will after segmentation, but that the remaining somites are densely packed in tip of the tail bud. Brief heat shocks at 37 and 38.5 degrees C reveal that within the first of these two zones, there is a graded sensitivity to the shock, with the primordia closest to the last-formed somite showing the greatest resistance. However, primordia within the densely packed tip (the packing zone) also appear resistant, or have sufficient time to repair the damage. We propose that once cells have left the packing zone, they undergo progressive patterning which renders them increasingly insensitive to the disruptive effects of heat shock, and culminates in rosette formation.