The 5 pairs of visceral arches of the head side have gradually degenerated when embry-onic length is 4 ~ 15mm . The oral plate ruptures to connect with the fore-gut .

胚长4～ 14mm，头侧5对鳃弓逐渐退化，口膜破裂与前肠相通，上下颌及舌已形成。