In a pennaceous feather, the barbs grow helically around the tubular feather germ and fuse on one side to form the rachis.Simultaneously, new barb ridges form on the other side of the tube.

正上，枝以螺旋形绕的芽，子的一侧接合一起形成轴，同时子的另一侧形成新的枝脊。