The First Fourteen Days of Human Life. Some of the analogies are stronger than others, but I ultimately agree with the authors that we can't deny personhood to a zygote just because it hasn't implanted yet, or because it is still capable twinning or fusion.

For me, the moral status of naturally developing human zygotes is pretty clear; a murkier matter is the status of unnaturally developing human blastocysts. For example, for my most recent adult Sunday school class, we watched a video of a lecture by Rudolf Jaenisch about the moral implications of his work with Alexander Meissner in which they "used nuclear transfer to derive mouse blastocysts from donor fibroblasts that carried a short hairpin RNA construct targeting Cdx2. Cloned blastocysts were morphologically abnormal, lacked functional trophoblast and failed to implant into the uterus. However, they efficiently generated pluripotent embryonic stem cells when explanted into culture." Assuming this procedure would work on human cells, would the result be a just a clump of cells, or would it be a very sick, very young person?

For sufficiently large N, a (non-gamete) cell from an adult human body can (in principle) be transformed, via N biochemical manipulations, into a genetically identical cell that is fully capable of implanting in a uterus and eventually becoming an adult human. So, where do we draw the line? At the risk of repeating myself, I think we will ultimately have to draw a lot of counterinuitive moral lines based on narrow genetic and epigenetic differences which cause major developmental differences. (Even then, some development differences will not be dichotomies, but rather differences of degree; a clear dividing line may be forever elusive in such cases.)

Regarding Jaenisch and Meissner's procedure, I honestly hadn't thought about it much between my October 2005 post and last Sunday. I was in favor of the idea last year, but coming back to the issue this week, I had doubts about the essentiality of the trophoblast, the precursor to the placenta which Cdx2 directs the formation of. Does preventing a functional trophoblast prevent a person, or create a very sick person? Going back to William Hurlbut's December 2004 paper for the President's Council on Bioethics, the first and second footnotes assuage my doubts. Key phrases: "studies confirm that normal trophectoderm is essential for normal embryogenesis" and "Cdx2 is essential for axial elongation in mouse development." If we really want to be on the safe side, then perhaps we should prevent gastrulation; Hurlbut cites evidence that this could be achieved by knocking out a gene called Hnf4.

Update: Let's spice things up with a concrete prediction: within 10 years, you won't be able to get NIH funding for research on human blastocysts unless you genetically/epigenetically make them incapable of implantation.