Four-cell Embryo Fossil (image) Virginia Tech Share Print E-Mail Caption A more-than 551 million year old four-cell embryo magnified using a scanning electronic microscope. The specimen is about 0.65 millimeter in diameter. Credit Shuhai Xiao Usage Restrictions May be used to illustrate reports about Shuhai Xiao's research. Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.