On the normal and abnormal development of the feather.

T H E 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 de­ scribed 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. In the ordinary feather (teleoptile) each barb rudiment was regarded as growing by the incorporation into it from below of cells in generators of the cylinder lying progressively, from tip to base, nearer and nearer to that unique 'dorsal' generator which will become the rachis. Thus, on the later splitting of the cylinder along a 'ventral' generator (opposite to that of the rachis) the barbs would slope toward the rachis, from above downwards, as, of course, they do in the familiar feather. Because of the morphology of certain feather defects, and of the shapes of the areas on some feather vanes showing a change in colour resulting from the injection of hormones (particularly of oestrone) into the bird, another view of feather development was advanced by F. R. Lillie in 1932. He supposed that there was a real tangential movement of actual tissue during the formation of barb ridges, from the 'ventral' to the 'dorsal' (rachis) generator of the cylinder. The tissue which he supposed to move tangentially in this way he considered to be made up of foci for the development of barb ridges, which formed barb distal to themselves all the time until they arrived at the rachis and became incorpor­ ated into it. This, the so-called theory of 'concrescence', has been modified from