By 11 weeks, there is some evidence of immunological competence-that is, some ability to fight infection. Also at this time of development one can note that the skin is comprised of both epidermal and dermal layers. Clearly, all of our current scientific data pertaining to human embryological and fetal development leads us to the inescapable conclusion that human personhood is indeed established at fertilization. One cannot begin to understand the beginning of human life without a proper philosophical understanding of the nature of human personhood. This assertion betrays a serious misunderstanding.
Potency means capacity for change and is found only in that which already exists.
In other words, potency and act are metaphysically inseparable. The human zygote is a human embryo and is a both potency and act. The human zygote is not followed by a series of new natures that represent a juxtaposition of different beings. The human person proceeds in his development in an unbroken manner. The person is human from conception and remains so throughout all of his change.