Chick embryo skin is a morphogenetic system easily accessible to experimental analysis. Like most other organs it is composed of two tissues, one epithelial, the other mesenchymal, of distinct embryological origin. The advantage of skin over other organogenetic systems is that its two components, epidermis and dermis, become superimposed upon each other at the periphery of the body early in development and stay in this situation throughout life, so that they remain readily attainable for experimental manipulation. Epidermis is a pluristratified keratinizing epithelium of ectodermal origin. while dermis arises from mesoderm and acquires a typical mesenchymal (connective tissue) structure. The two tissues are linked to each other by extracellular matrix, part of which, the basement membrane, mostly of epidermal origin, develops underneath the basal pole of basal epidermal cells, the rest of it being of mesenchymal origin and constituting the interstitial dermal matrix. Skin produces various types of adnexa, such as cutaneous glands and keratinized appendages. The latter in birds comprise feathers and scales, inaddition to beak, comb. spur. claws and wattle. Inthe present brief review, discussion will be restricted to the development of feathers and scales.

Several fundamental questions have been asked regarding the development of those two types of cutaneous appendages. Some of them have received partial answers. They refer to the origin of skin and appendages, to the establishment of the feather or scale pattern, to the mechanisms whereby individual feathers or scales are constructed (Sengel, 1976a. b). The explanation of the origin of skin is pretty straightfol" Nard, although not entirely solved. During gastrulation and neurulation, presumptive mesoderm reaches an inside location where the outer part of its somitic and somatopleural components becomes associated with the overlying ectoderm. Interspecific quail/chick transplantation experiments have shown that in the dorsal and dorso-Iateral regions, presumptive dermal cells arise from the dermatomal part of the somitic mesoderm, while in the lateral and ventral body parts they originate from the somatop! eurallayer of the lateral plates (Mauger. 1972a). In the head region, the majority of dermal cells arise from neural crest cells. Once the entire previously cell-empty space beneath the ectoderm is populated by prospective dermal cells. the integument has acquired morphogenetic properties, the expression of which can lead to the formation of cutaneous appendages. By this timecorresponding in the chick embryo to approximately 4 days of incubation-although skin as such is not yet individualized, particularly because dermis is not yet histologically distinguishable from the underlying subcutaneous mesenchyme, the presumptive integument. made up of the ectodermal cover, the ecto-mesodermal junction and the predermal mesenchyme, is able to give rise to cutaneous appendages when isolated in a suitable nutritive environment. such as the chorio-allantoic membrane of the chick. Thus. sometime between gastrulation and 4 days of incubation, the