In response to the appearance of such variants in the cell population the concentration of the toxic chemical in the growth medium is increased, thus keeping the selection pressure constant.
为应对细胞污染中出现的变异，有毒物质的浓度是逐步增加的，以此保证筛选力度的稳定。
2
But the medium is hard to dismiss as a reporters' tool, particularly in countries where cell-phone networks reach farther than the Internet.
但是这一媒介作为一种报道的工具很难抛弃，特别是在那些电话网络比因特网覆盖更广的国家。
3
Two or three channels imprinted onto the plastic square contain either a normal cell growth medium or a chemical under study, such as growth factor.