Feather pattern development.
Experiments on skin development in amniotes have shown conclusively that the pattern of cutaneous appendages is determined by the dermis.  The following concept of feather pattern formation is proposed.  The diameter of feather rudiments and the dermal cell density inside the dermal feather condensations are genetically fixed, region-specific parameters.  The first feather rudiment is established along the middorsal line (in the lumbar region) as soon as a sufficient width of dense dermis is available.  This occurs at about 6 1/2 days in the chick embryo.  The next rudiments are formed in front of and behind the first one, within the longitudinal band of dense dermis.  They are laid down as close as possible to the first one, given rudiment diameter and dermal condensation cell density.  In other words, as they form, the middorsal rudiments are tangent circles.. Later they become separated from one another due to longitudinal growth.  As progressively more dense dermis becomes available to the sides of the middorsal row of rudiments, new rudiments are being laid down lateral to the middorsal row, in successive longitudinal rows, until, by 9 or 10 days, all the available dense dermis has been used up.  Again, the rudiments in the lateral rows, given their diameter and dermal condensation cell density, develop as close as possible to those of the preceding row, i.e. tangentially to two neighbouring rudiments of the preceding row, thus generating a hexagonal pattern.
D000818:Animals; D002642:Chick Embryo; D046508:Culture Techniques; D004372:Ducks / Q000196:embryology; D005241:Feathers / Q000196:embryology*; D008116:Lizards / Q000196:embryology; D051379:Mice / Q000196:embryology; D008954:Models, Biological; D011995:Recombination, Genetic; D012867:Skin / Q000196:embryology*; D013045:Species Specificity; D013997:Time Factors