1. For a slightly fuller account of these phases see C. H. Waddington Principles of Development and Differentiation (Macmillan: New York 1966) and Proc. Roy. Soc., B, 164 (1966), 219.
2. C. H. Waddington in Kitching (ed.), Recent Development in Cell Physiology (Butterworth: 1954), p. 105.
3. J. B. Gurdon Endeavour 25 (1966), 95.
4. C. Pavan and R. Basile Science, 134 (1966), 1956.
5. F. M. Ritossa and S. Spiegelman Proc. Nat. Acad. Sci, 53 (1963). 737; M. L. Birnstiel et al., Nat. Cancer Inst. Monog., 23 (1966), 431.