My comments were quotes (with a little explanation) from Norman M. Ford's When did I begin? Conception of the human individual in history, philosophy and science (Cambridge, UK: Cambridge University Press, 1988), the most thorough book on this question that I have been privileged to read. I am not, of course, certain that he is correct, but he at least deserves a reading. Ford is apparently a Catholic believer, with expertise in embryology, and also with knowledge of theology and philosophy. I'm going to repeat the quotes that I used in my comments here, in my own blog:
A human person cannot exist before the formation of a distinct living ontological individual with a truly human nature that retains the same ontological identity throughout successive stages of development. (xvi) (See the Wikipedia article on Ontology, if you need to.)
It should not be assumed that a genetically unique human zygote is the same ontological individual as the resulting blastocyst, definitive embryo proper, fetus and child, notwithstanding the continuity of the same biological identity at every stage of development. Human twins that are genetically identical are neverthless different ontological individuals. Furthermore, analysis of the evidence shows that early embryonic cells inside the zona pellucida lack the requisite unity to constitute a single ontological entity. Each is totipotent. They appear to have too much independence of behavior to constitute one individual. This alone would preclude them from being a human individual until the multiplying cells formed themselves into a single multicellular human body. Furthermore, experiments with mice show how single cells taken from three separate early mouse embryos can be aggregated to form a single viable chimaeric mouse embryo. In this case the resultant individual mouse certainly did not begin at the zygote stage. This suggests that perhaps in the normal situation the proliferating and developing cells amalgamate at a later stage to form the definitive individual body, be it that of a mouse or a human individual. (xvii)
With the appearance of the primitive streak after the completion of implantation and about 14 days after fertilization identical twinning can no longer occur. This is when the human body is first formed with a definite body plan and definitive axis of symmetry. Prior to this stage it would seem to be quite unreal to speak of the presence of a distinct human individual. This suggests that before this time genetically human embryonic cells could not develop into a human individual with a true human nature and a continuing ontological identity. If I am right, the early embryonic cells could not constitute an actual human individual. Instead they would have the potency to form one or more human individuals. It seems that the biological evidence leads to the philosophical conclusion that a human individual, our youngest neighbor and member of the human community, begins at the primitive streak stage and not prior to it, but most certainly by the stage of gastrulation when the human embryo's primitive cardiovascular system is already functioning and blood is circulating. (xviii)
In giving her moral teachings, which I personally accept, the Church generally refers to the zygote or the product of fertilization as a human being, a human subject with rights and even a human person but without intending to commit herself to a statement of philosophical character. In other words, the Church has gone as far as she possibly could without expressly declaring that the zygote is a human individual in the philosophical sense of a human being or human person, personally identical with the fetus, future infant, child and adult. Consequently though different stages of development of the human individual are recognized, the Church places them all on the one ethical level. . . . (p. 62) Ford is saying, here, that the Catholic Church's well-known opposition to abortion, at any stage of development, is on ethical grounds, not on the grounds that a zygote or very early embryo is philosophically equal to a baby or adult.
There's a lot more thought-provoking material in the book, but I don't think it's ethical for me to quote any more of it here.
Ford's answer to the question posed by his book's title, and that of this post, would be, "no earlier than about 14 days of development."