Different kinds of polarity form, there's a head, and there's a tail, there's a back, there's a front, there's a left side and there's a right side.

不同的极性开始形成,有头端,也有尾端,有后面,也有前面,有左侧,也有右侧

耶鲁公开课 - 生物医学工程探索课程节选