The initiation of limb bud outgrowth in the embryonic chick

Changes in the rate of cell division that occur during the initiation of outgrowth of the embryonic chick wing have been investigated by determination of changes in the labeling index in the wing region and the flank region. It has been found that the labeling index changes very little in the wing region between stage 16 and stage 20, but decreases rapidly in the flank region during the same period. The decrease in labeling index in the flank region results from a change in the proliferative index in that region from 100% at stage 16 to 75% at stage 19. This change in the proliferative index in the flank cannot be easily correlated with differentiative changes in the flank, but correlates very well with morphological changes.