来自《耶鲁大学开放课程：生物医学工程探索 第6集 第6集》

• 
  
  there's a sheet of cells that forms an outer lining,
  一层细胞构成了外形轮廓
• 
  
  and what is most noticeable is this cavity,
  最明显的结构就是这个空腔
• 
  
  this fluid filled cavity which begins to develop.
  这一充满液体的空腔开始发育形成
• 
  
  Well, this stage of development is called the blastocyst
  我们称此发育阶段为囊胚期
• 
  
  and it's at this stage late in this blastocyst stage
  这是在囊胚期的终末阶段
• 
  
  that the developing embryo implants in the uterine wall,
  发育中的胚胎在子宫壁上着床
• 
  
  and there begins to form an interface with the mother
  并开始形成与母体的接触面
• 
  
  so that it can be nourished during further development.
  从而汲取营养以供进一步发育
• 
  
  The cells of this surrounding sheet
  环抱在周围的这一层细胞
• 
  
  have become different in some way
  已经开始逐渐分化
• 
  
  and they develop into the placenta
  最终成为胎盘
• 
  
  and the extra embryonic tissues.
  及额外的胚胎组织