The first model of three-dimensional organization of a two-layered epithelia consisting of AB hexagonal cells. Left part of the Fig. shows a shape and mutual arrangement of cells. The superior and inferior parts of the model have the same construction but opposite orientation. The (a-c) part in the middle of the Fig. is the mosaic, corresponding to the tangential sections. It demonstrates the neighbouring cells at the different levels of the layer. The cellular shape of apicobasal section (across the cellular centers) of the model is presented on the scheme (d). Right part shows a real pattern of cell connections at the corresponding levels. From up to down it is shown:

• a) an apical surface of grass frog larva epidermis. SEM;
• b), c) Semi-thin tangentional sections of epidermis at the level close to middle. The small and large triangle cell profiles and the intersections of six cell edges give a pictures comparable to mosaic b and c respectively;
• d) Semi-thin apicobasal section of two-layered epidermis. Sharpened cells form a characteristic interlayer boundary with an "accordeon" appearance;
• e) An internal surface of basal layer, which are seen after removal of the superior cell layer. The apexes of the basal cells are seen as the trigonal pyramids. SEM. From Savostyanov & Grefner 1998.

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