The zygote or this fertilized egg is a completely undifferentiated cell.

受精卵,或者说已受精的卵子,是一个完全未分化的细胞

耶鲁公开课 - 生物医学工程探索课程节选

Those B-cells also mature, they differentiate and that's what's shown on this slide here is the differentiation of those immature B-cells into mature B-cells.

这些B细胞也会变成熟,并且进行分化,这都展示在幻灯片上了,未成熟的B细胞分化为成熟的B细胞

耶鲁公开课 - 生物医学工程探索课程节选