The base bodies of dorsal cilia expand gradually and in the mean time the peripheral microtubules in basal bodies apart gradually from each other forming a plum blossom-like arrangement.

• 背纤毛基体逐渐膨大，基体内的周围微管散开成梅花形，之后在相应皮层区发生由颗粒组成的条带形原基。