干背

step back───后退，退后；无意义

steps back───后退，退后；无意义

storm back───风暴回来

set back───推迟；使…受挫折；把…往回拨

bites back───咬回；急忙收回要说的话；抑制

dated back───追溯到；回溯至

dates back───追溯到；回溯至

screw back───退回

shell back───n.老水手

If you've got extra cells of one type and need another, why go all the way back to a stem cell?───如果你有一种类型的多余细胞，还需要另外一种类型的细胞，干嘛还要回到干细胞上去呢？

Cut out a few leaf shapes from green felt and hot glue a small length of the stem wire to the back of each.───绿色毛毡布切割出叶子的形状，背面粘上一小段铁丝。

President Bush says he will not allow legislators to roll back limits on stem cell research he set in 2001.───布什总统表示他不会允许国会议员们取消他在2001年对干细胞研究所做的限制。

Pull stem back out to full - open position.