A SCANNING ELECTRON-MICROSCOPIC STUDY OF TONGUE DEVELOPMENT IN THE FROG RANA-PIPIENS

Feeding behaviour changes drastically during metamorphosis as larval suction feeders become adult lingual feeders. In order to understand this transition, the general morphological development of the floor of the buccal cavity in embryonic and larval Rana pipiens was studied, up to the completion of metamorphosis, by scanning electron microscopy. Rana pipiens specimens were collected, anaesthetized with tricaine methanesulphonate, staged by the methods of Shumway and Taylor and Kollros, and fixed in 0.1 M phosphate-buffered 2.5% glutaraldehyde. The oropharyngeal floors were dissected and routinely prepared for scanning. The late embryonic period (Shumway stages 21-25) is marked by the appearance on the oropharyngeal floor of two midline premetamorphic lingual papillae (PMLP), located on the second branchial arch just caudal to the hyomandibular groove. The larval tongue anlage, which incorporates PMLP along its anterior border, does not appear until stage V of the premetamorphic developmental span (Taylor-Kollros stages I-XI). Prometamorphosis (stages XII-XIX) is marked by the incorporation of the larval tongue into the adult tongue, the disappearance of the PMLP, and the appearance of the true tongue papillae. The metamorphic span (stages XX-XXIV) marks further rapid growth and differentiation of the adult tongue.