We analysed limbless tadpoles in the ontogenetic range 23-27, marked by development of operculum and labial teeth (
[Gosner, 1960](m7604a01:B15)). Epidermis maintains a stable structure in these stages, until cutaneous glands develop during toe morphogenesis ( [Delfino et al., 1991](m7604a01:B2)). Tadpoles of Pelobates cultripes - six specimens - and Phyllobates bicolor - eight specimens - were collected near Ciudad Real (Spain) and Medelin (Colombia), respectively, and sacrificed with 0.1% chlorobutanol. Skin strips, 9 mm2, were fixed in the [Karnovsky (1965](m7604a01:B18)) solution, postfixed in 1% OsO4 (both in cacodylate buffer) and embedded in Epon 812. Semi-thin sections of 1.5 µm thickness (for the LM) were stained with buffered (1%, borax) toluidine blue, ultrathin sections with a hydroalcoholic saturated solution (25 mg/ml) of uranyl acetate followed by alkaline lead citrate (2 mg/ml). Ultrathin sections were observed (80 KV) with a Siemens 101 TEM.