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Abstract

Scale development in the chick embryo can be considered as consisting of two major phases. First, a definitive scale ridge is formed from a simple two-layered epidermis. This phase consists of five stages: preplacode, placode, asymmetrical placode, hump stage, and definitive scale ridge. The second phase involves the elongation of the definitive scale ridge to give rise to the overlapping scales of the adult bird. During elongation a new (inner) epidermal surface is formed, which is histologically less differentiated than the outer epidermal surface. A timed-sequence study has shown that the placode gives rise mainly to the outer epidermal surface of the definitive scale ridge, while the inter-placode region becomes the thinner epidermis at the scale apex.