One of the most interesting results of research that Fr Fleming reported today was on the differentiation that is present in the human embryo from conception. You can read a summary of the results of this research in an article from the news features section of Nature: Your Destiny, From Day One.
This research shows that the understanding of the embryo in its early stages as a featureless bundle of cells is no longer tenable. It also fatally undermines the old notion that "personhood" can only come about after the potential for twinning has been passed (about 14 days). Far earlier than this, the embryo is differentiating rapidly. Twinning could best be seen as a type of natural cloning where both the original embryo and the "younger" twin are indisputably human lives from the moment they begin to exist as such.