The primitie mouth, or stomodeum, is a slight depression on the surface of the ectoderm.

原始口（或口凹）是外层表微凹陷。

Bilateral oal thickenings of the surface ectoderm, called nasal placodes, then deelop on each side of the inferior part of the frontonasal prominence.

外层表呈双边卵圆形增厚形成鼻板，然后发展成为额鼻隆起下半部分。