All interpretations of osteological characters are based on specimens cleared in potassium hydroxide and stained with alizarin red.
"A Review of the Middle American Tree Frogs of the Genus Ptychohyla"
William E. Duellman
But this indigenous product has been ousted by alizarin, a colouring agent made from coal-tar, which is imported from Germany, and is about thirty per cent cheaper than the native dye.
"The Tribes and Castes of the Central Provinces of India Volume II"
R. V. Russell
Osteological data were obtained from specimens that were cleared in potassium hydroxide, stained with alizarin red, and stored in glycerine.
"Middle American Frogs of the Hyla microcephala Group"
William E. Duellman M. J. Fouquette