Publisher Summary This chapter discusses the cell differentiation of metamorphic transformation of larval organs. The larva-specific organ—the tail—is focused; the larva-specific organ is destined to disappear eventually at metamorphosis. The chapter describes the process of tail breakdown, morphologically and biochemically, and the mechanism of the breakdown is considered in relation to both epithelium–mesenchyme interactions and thyroid hormone (TH)-induced cell death. The larva-to-adult organs are surveyed, targeting the skin, intestine, and liver. The chapter also discusses the mechanism of action of TH in metamorphosis, with special reference to receptors for this hormone. The morphological changes occurring in apoptotic tissues with the activities of hydrolytic enzymes, putting emphasis on collagenase and protease T1 are correlated. Thyroid hormone directly induces the death of larva-specific cells by suppressing DNA synthesis. The larval skin appears to be the transformation of larval basal cells into adult germinative cells. Tadpole liver contains both larval and adult hepatocytes, which have different cell lineages. Cell-culture techniques should be developed and elaborated for cells of tadpoles to obtain decisive evidence for either the replacement theory or the transformation theory.