Fig. 3. Electron micrographs showing details of stratum corneum and permeability barrier of terrestrial vertebrates. (A) Section through a portion of cocoon of a burrowing hylid frog, Pternohyla fodiens. The layers of squamous epidermal cells are separated by granular extracellular materials in the subcorneal spaces. Scale bar, 500 nm. Reproduced with permission (Ruibal and Hillman, 1981). (B) Section through mesos layer of snake epidermis (Natrix natrix), which is the recognized permeability barrier of squamates. Laminated lipids occur between the darker bands of keratin layers. Scale bar, 100 nm. Courtesy of Lukas Landmann. (C) Section through stratum corneum of human skin. Lipids (unstained) occur between the distinct layers of keratin. Scale bar, 200 nm. Courtesy of Gopi Menon. (D) Section through epidermis of a canary, showing nucleated layers as well as stratum corneum (top). Lipids occur between the distinct layers of keratin toward top of figure. Note the multigranular bodies (source of lipids; arrows). Scale bar, 200 nm. Courtesy of Gopi Menon.