12 September 1997
Vol 277, Issue 5332

About The Cover

The radiation of animal life around the Precambrian-Cambrian boundary (the "Cambrian Explosion") was a major event in the history of life. The cover shows phosphatized fossil embryos (~0.5 millimeters in diameter) of two animals from that time, a probable cnidarian (rear) and a segmented worm (front). Both appear to have been direct developers (no larval stage) with large, yolk-filled eggs. The fossilization of soft embryos opens windows on the study of developmental biology of extinct organisms. See 1645. [Image: S. Bengtson and Y. Zhao]