In the Literature

Development of Plumage Color Patterns. For the experimental biologist investigating the factors that control the processes of development, a major problem has been the selection of well defined criteria of embryonic differentiation, which, in response to experimental treatment, undergo definite, unequivocal changes. The striking and intricate plumage color patterns of birds, long recognized as important tools by students of speciation and evolution, afford an unexcelled system for the experimental analysis of problems in embryonic differentiation. Willier (1942, 1948) has emphasized the advantages of the feather papilla in this respect: (1) it appears regularly after plucking the feather and is thus readily accessible for experimental study, and (2) it has many characteristics of a developing embryonic organ, for example, axial organization, inductor action, and physiological gradients in response to various stimuli. Willier also has reviewed the recent advances in our knowledge of the genetic and environmental (particularly hormonal) control of development contributed by studies on the differentiation of plumage pigment patterns in the fowl. Most widely studied have been a group of pigments, called melanins, that are probably derivatives of the amino acid, tyrosine, and that range from yellowish brown to black, and that are deposited in the feather in the form of granules by branched pigment cells, the melanophores. These highly specialized cells have their origin in the embryonic neural crest and migrate while in an immature, unpigmented form (melanoblasts) to the feather papillae of the skin. (For extensive review of this phase of the problem, see Rawles, 1948.) Of the components essential to melanin pigmentation of the feather, only the melanophores have an extrinsic origin. This condition makes it possible to produce feathers which are characteristic, in every respect, of the pigmented breed from which they were derived, except for a complete absence of color. This production is accomplished by transplanting the embryonic limb bud of a pigmented breed, prior to the entrance of the melanoblasts, to the coelom of a White Leghorn embryo, an environment essentially free of pigment cells, where normally shaped, but unpigmented, feathers are formed. The melanoblasts, under certain physiological conditions as yet poorly understood, invade the feather papille, and are there subject to the action of environmental factors imposed by the growing papillae. The specific response of the pigment cells is, however, governed primarily by their genetic makeup. For example when neural crest cells of one breed are grafted into an embryo of another breed, the resultant color pattern in the host feather invariably resembles that of the donor breed. The analysis of Willier and Rawles (1944) who tested the response of melanophores derived from embryos that have