Development of beaks of Rana pipiens larvae.

Rana pipiens larval beaks of consist of column cells, sheath cells and basal cells which supply cells to column and sheath. Each column consists of disk-like precone cells, cone cells and keratinized cone cells; they are cells in different stages of the process of keratinization. Breaks first appear externally at embryonic stage 24. Epidermal cells align at the tip of the jaw at stage 21. They increase in number and change in shape. Keratinization starts at stage 23. By stage 24, the apical column cells are keratinized and the histological organization is set for the whole larval period. During the larval period, the numbers of column cells increase until stages VIII or IV, stay relatively constant during mid-larval stages, and decrease at late larval stages. The beak is completely shed at stage XX. The widening of the beaks goes on during the entire larval period. Along the cutting edge of the jaw the tooth spikes increase in number and in individual width as the animal grows older and larger. Thyroid hormone causes a premature reduction of the column cell number and a precocious beak loss. The loss of break at metamorphic climax is thyroid dependent event.