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
,
枝以螺旋形绕著管状的
芽生长,
管子的
侧接
起形成
轴,同时
管子的另
侧形成新的
枝脊。