SummaryI. Feather papillae are regarded as individually persistent throughout life, from the time of their origin between the sixth and ninth days of embryonic life. In each feather tract the order of origin is significantly related to dynamic field properties of the tract, such as growth rates, degrees of asymmetry, etc.2. Recent work on the anatomy and development of the rhachis with special reference to barb relations and the independent origin of the central rhachis is considered; also the development of the after‐feather and of the pulp and its blood supply.3. By plotting axial growth and isochrones on the vane of the feather it is possible to relate any definitive locus to its time and place of origin in the germ, and hence to the conditions of its determination.4. The papillae of definitive feathers may be transplanted, or subdivided, or parts of papillae of different tracts may be recombined. The results of the operations show that papillae have a definitive bilateral organization, like amphibian eggs, for example; that they are tract‐specific; and that a dorsal field, including the central rhachis, exerts a dominant influence on development. Various abnormalities of feather form produced may be interpreted. Twins and chimaerae may be produced. Abnormalities artificially produced persist through successive generations of feathers from the same follicle. The mesoderm of the papilla is regarded as the seat of the initial inductions.5. The melanophores of feather germs are derived from the neural crest. When transplanted to foreign fowl hosts they determine the donor pattern in the host, as, for instance, barring in a white host. The genotypic composition of the donor melanophore is the controlling factor in both colour and pattern of the host feather in these experiments in all breeds of fowl examined.6. The growing feather germ is a very sensitive indicator; reactions may be read by pattern modifications in the finished feather and referred to their locus of origin in the germ (cf. no. 3).7. The thresholds of reaction of feathers to female hormone increase in proportion to rates of axial growth. This is very conspicuous with reference to the discontinuous rates of different tracts, and is also to be observed with reference to gradations of growth rate within a single tract.8. Within the individual feather germ of breast and saddle tract, threshold of reaction to female hormone and thyroxin rises from dorsal to ventral loci of the collar. These transverse gradients produce transverse patterns on the vane of the feather varying in form according to rise and fall of hormone in the blood stream by absorption and excretion after injection: the principle of rate of reaction which is inversely proportional to threshold is also a factor in the determination of pattern.9. The principles of physiology of development may be applied to some problems of physiological genetics, as, for instance, in the case of barred patterns.