Fig. 316:Scheme of an advanced feather structure partially cut open (according to Peters 2001; from Junker & Scherer 2006). The follicle develops by sinking of a papilla into the skin. Thus cells multiply in a ring around the feather structure. The outer skin layer releases cells which form parts of the feather. The feather develops as a tube in the initially closed sheath. While growing the outer skin layer splits into columns which later form the barbs. The development of the feather is completely different to the development of reptilian scales