With Patrick Lee. The New Atlantis, Summer 2006.

Excerpt:

In the debate about the moral standing of human embryos, some defenders of embryo-destructive research have claimed that human embryos are not human beings until implantation (i.e., when the embryo attaches to the uterus, approximately six days after fertilization), and others have claimed that they are not human beings until gastrulation (i.e., when the possibility of twinning no longer exists and the primitive neural streak first appears, approximately 14 days after fertilization). These claims have been repeated by policymakers, scientists, and bioethicists alike, yet they fly in the face of the embryological evidence. Seeing why will put the embryo research debate on a more solid biological footing.

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The New Atlantis