There's a cluster of cells here, there's a sheet of cells that forms an outer lining, and what is most noticeable is this cavity, this fluid filled cavity which begins to develop.

有一簇细胞,一层细胞构成了外形轮廓,最明显的结构就是这个空腔,这一充满液体的空腔开始发育形成

耶鲁公开课 - 生物医学工程探索课程节选