1. Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, United Kingdom
2. The Wellcome Trust/Cancer Research UK Gurdon Institute, University of Cambridge, Cambridge CB2 1QN, United Kingdom;
3. Division of Germ Cell Biology, National Institute for Basic Biology, National Institutes of Natural Sciences, Myodaiji, Okazaki 444-8787, Japan
4. Department of Basic Biology, School of Life Science, Graduate University for Advanced Studies (Sokendai), Myodaiji, Okazaki 444-8787, Japan
5. Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Cambridge CB3 0WA, United Kingdom
6. The Wellcome Trust/Medical Research Council Stem Cell Institute, University of Cambridge, Cambridge CB2 1QR, United Kingdom