26 January 2007
Vol 315, Issue 5811

About The Cover

Cover image expansion

COVER Mouse zygotes that are developing parthenogenetically (without fertilization) with a single polar body and two pronuclei. These zygotes develop into blastocysts, from which embryonic stem cells can be harvested. Such cell lines can provide a source of immunologically compatible tissues for transplantation. Zygote outer diameter, 70 to 80 micrometers. See page 482. Image: K. Kim, K. Ng, A. Yabuuchi, and G. Q. Daley