Abstract
Rana pipiens tadpoles were injected with ^3^Hβthymidine at different stages to label basal cells of the horny beaks, the keratinized oral specializations of many anuran tadpoles. Cells in the column and sheath of the beak are derived from the basal layer. Movement of the labeled cells in beak columns can be traced, permitting calculation of the rate of cell addition into the columns. With the counts of the total number of column cells and cell addition rate, the rate of apical cell loss from a column throughout the larval stages can also be calculated. The rate of cell addition decreases steadily from stage III through stage XX, in all portions of both beaks. The rate of change of these column cell addition rates is similar among middle and lateral portions of both upper and lower beaks. There is a relatively constant rate of cell loss until stage XII or XIII. It then increases sharply, and the beaks are completely lost at stages XIX or XX. The longer, lower beak columns have a much faster cell loss rate than do the upper beak columns at late larval stages. Life spans for cells that move to the sheath may be about oneβhalf those for cells that enter the beak columns. It is suggested that thyroid hormone accelerates the rate of column cell loss.