Given the growth rate throughout an embryonic rudiment, one can calculate how its parts must shift as it gets bigger, and so plot fate maps showing their future positions. The calculation is done here for the chick wing-bud from stage 18 to stage 25, using published data for mitotic index and cell packing density, and new measurements of size and shape. Two main findings are: (1) the wing-bud in this period elongates almost uniformly along itsproximodistal axis. (2) Each limb segment derives from the tissue generated by one cell division cycle in the ‘progress zone’ at the tip.

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