AT THE 1990 MEETINGS for the British Association for the Advancement of Science, an experimental embryologist expounded an expert's view to a lay audience.' Martin Johnson was concerned to demonstrate the continuity of biological process. A person's birth begins with primitive gametes laid down when one's parents were embryos in the grandparental womb. Subsequent development depends not only on genetic coding but on extragenetic influences that operate on chromosomes from the start; these include stimulation from material enveloping the egg,2 as well as nutritive and other effects derived from placenta and uterus. It was a powerful origin story,3 especially in the context of current legislative decisions with respect to the Human Fertilisation and Embryology Act (1990). Here, however, the problem has been to formulate discontinuities between developmental phases. The House of Commons decided that research on human embryos is permissible up to fourteen days, by which time, among other things, the pre-embryonic material is now discernably divided into those cells that will form the future embryo-fetus and those that will form the placenta. The Secretary for Health was reported as saying that status as an individual could begin only at the stage where cells could be differentiated.4 Yet while biology appeared to provide an index,5 the further problem of personhood raised the same notion of continuous process. Another member of the Commons pointed out: "It is a very difficult matter to say at what stage do you have a citizen, a human being. At various stages fresh rights are acquired."' Rights can only be acquired of course, in this view, if there is an individual person to bear them.7 Here are experts informing lay persons (the BAAS talk), experts