When first formed, the embryo is only half a millimetre long.胚胎最初形成时仅半毫米长。The rainfall there averages 300 mm a year.那儿的年降雨量平均每年为三百毫米。The silicon chips are less than a millimeter thick.这些矽片厚度不足一毫米。A micron is one-thousandth of a millimetre.一微米等于千分之一毫米。