1. Studies carried out on the stage 14 chick embryo indicate that in normal and in colchicine-treated embryos mitotic frequency is greatest towards the posterior tip of the elongating pronephric duct.

  2. Although the mitotic frequency in the duct tip is high compared to that in the duct at more cephalic levels, it is similar to that in other mesodermal regions at the same level.

  3. Distortions of the embryo which occur with colchicine treatment suggest that migratory activity is prominent towards the duct tip, and emphasize the dependence of normal morphology of the duct on tensions in the blastoderm.

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