Avian scale development. Absence of an "epidermal placode" in reticulate scale morphogenesis.

Timed-sequence studies have shown that reticulate scales on the ventral footpads of birds do not undergo "epidermal placode" formation during their morphogenesis, but arise as symmetrical evaluations similar to the scales of snakes and lizards. Unlike the scutellate scales on the dorsal surface of the foot, in which the formation of an "epidermal placode" and its subsequent morphogenesis result in disticnt outer and inner epidermal surfaces, the reticulate scales elaborate only one type of epidermal surface.