Christopher Graham has conducted meticulous and highly original studies on early mammalian embryos. He devised an effective way of producing parthenogenetic activation of mouse eggs in vitro, the first to allow a choice between haploid and diploid development. This work established that neither the maternal tract nor the fertilising sperm is necessary for histogenesis, since even haploid parthenogenones produce all tissue types. His studies on the cell cycle were the first to show that genes may be expressed in mammals even at the two-cell stage. He demonstrated, by ingenious reaggregation experiments, that at least the first stages of embryonic differentiation are regulated by cell position within the embryo rather than by inherited morphogenetic determinants. This finding, termed by him ‘inside–outside’ differentiation, is central to our present concept of early development. His ongoing studies of teratocarcinoma cells as a model for the normal embryo continue to emphasise the importance of the microenvironment in directing differentiation.