The feather is the most elaborate of all the derivatives of the Malpighian layer. The production of so complex a structure from so simple a layer of cells is a challenge to the developmental anatomist and has been studied for many years. Historically two main views of the fundamentals of feather development may be distinguished. In 1889 Davies published a very full account in which he described the growth as being essentially a simple lengthening, by increment at its base, of an ectodermal cylinder surrounding a mesodermal core. He was supported in this view by Strong (1902). Both of these authors thought of the barbs developing in the ectodermal cylinder of the protoptile as being each added to from below by the incorporation of cells always in the same generator of the cylinder, these barbs thus being parallel, with no rachis.

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