Bioethical debates such as those surrounding the manipulation of human embryos are often based on metaphysical assumptions that lack a foundation in the natural sciences. In this paper we support a gradualist position whereby the embryo progressively takes on the form and associated ethical significance of a human being. We support this position by introducing a concept of biological structure or form to show how the gradualist position has its metaphysical foundations in modern biology. The conceptual basis for form and (...) structure are outlined and their compatibility with and basis in current empirical biology is demonstrated by some recent advances in our understanding of the processes of development from single cell to organism. We then briefly explore the ethical significance of accepting a form or structure based conception of biology for the status of the early embryo. (shrink)