Skin: Frog skin:
True Colour on Clear Plastic: Brown Tree Frog (Litoria ewingii)
Retired Professor of Anatomy, Ian Gibbins, has kindly shared his thoughts with us to help people learn:
Many tadpoles also have transparent skin and you can see their heart beating, their guts, and more. In some small species, you can see through the cartilage of their skull and see the brain, optic nerve roots and more.
The tadpoles of Litoria ewingi often have iridiophores in their pericardium, the membrane around the heart, that makes them look gold or silver.
And it is actual skin. It contains the same basic layers as our: epidermis overlying dermis, with pigment cells sort of scattered / sandwiched in between. There also are glands in the dermis that produce mucus (equivalent to sweat glands in us). However each layer is very thin - only a few cells, as I recall - the the epidermis is only marginally keratinised. (The keratin provides internal strength to the epidermal cells, and also contributes to water-proofing.)
The skin of tadpole tails was used to directly image the growth of nerve fibres to the skin and muscles, as well as directly watching the process of regeneration in re-growing tail tips. My PhD started off learning how to do some of this, mostly using Limnodynastes.