Metamorphic changes of dermis in skin of frog larvae exposed to thyroxine.

Skin in the tadpole of anuran larvae prior to metamorphosis is composed of two layers of epidermal cells (Leeson and Threadgold, 1961; Chapman and Dawson, 1961; Edds and Sweeny, 1960, 1962), a laminated basement lamella, and a layer of dermal mesenchyme cells apposed to the underside of the basement lamella. An adepidermal space and membrane (Salpeter and Singer, 1959, 1960) separate epidermis from the dermal basement lamella. Weiss and Ferris (1954) first observed that mesenchyme cells migrate into the basement lamella as metamorphosis begins. Invasion of the lamella has been reported to begin about Taylor-Kollros stage XI or XII (Kemp, 1961b) and to proceed until mesenchyme cells move entirely through the lamella and cause it to become detached from the adepidermal membrane at stages XV-XVI. Subsequently a population of cells constituting the stratum spongiosum develops beneath the adepiderma1 membrane and the basement lamella becomes the deeper-lying stratum compacturn or “dermal lamella” of the adult dermis. New polymerization of collagenous fibers and ground substance results in deposition of the adult basement membrane between the adepidermal membrane and the stratum spongiosum. Generalized metamorphic changes can be induced in amphibian larvae by exposing them to thyroxine or other thyroid hormones (Kollros, 1959; Pitt-Rivers and Tata, 1959). Local metamorphic changes can be induced by implanting pellets of thyroxine mixed with