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.

，枝以螺旋形绕著管状的芽生长，管子的侧接合形成轴，同时管子的另侧形成新的枝脊。