One thing about me that may help you ‘read me’ effectively: my portal of entry into philosophy was entirely through Aristotle and through Aquinas’s comments on Aristotle (or on Boethius). Thus I’ve always had not only a comfort in thinking about living things as acting for the sake of an end (gasp!), but even more; I start by thinking that living things are acting for the sake of an end. Final causality drives the living world, for me. So when I direct my thinking towards embryology I naturally find myself thinking, “how does this or that change that is produced in the embryo help its homeostasis while at the same time help it to produce new structures and organs through which it will prepare itself to survive in new environments, and even surmount them?”
I just grabbed my De anima (the Apostle translation): I’m beginning to wonder whether there is a deeper disagreement between me and the sorts of philosophers you are engaging with; maybe the problem isn’t about unity or continuity over time, or whether the ‘one’ zygote ceases at cell cleavage and we then have ‘two’ cells, neither of which was the first cell (hence discontinuity, and the impossibility of personhood). Maybe the real question is: “what is life for these philosophers?”
In De anima 2.1 (412a29–412b1) Aristotle defines soul (the principle of life) as: “…the first actuality of a natural body with the potentiality of having life; and a body of this kind would be one which has organs.” He then analogizes with respect to plants, noting that their leaves do one thing, while their roots (like mouths) do other things. Finally he streamlines the definition so that it will apply to all living things: “the first actuality of a natural body which has organs.”
“[W]hich has organs…” Aristotle has what seems to be an absurdly broad notion of ‘organ,’ but of course the word organ (organon) means ‘instrument’ or ‘tool.’ And a natural body that will be the source of agency both within itself and without will have to have varying organs that are suited to the needed work: hard parts like bones for structure, soft tissues for muscles whose elasticity allows them to expand and contract, etc. To live is to act through heterogeneous parts.
I say all this because the language of philosophical discussions on immediate personhood (the conception thesis) is conducted in the dry language of number (one, two, fewer, more, division) and continuity (duration, time, succession). I.e., it’s almost as though we are speaking about arithmetic (the science of discrete quantity) and a kind of temporal geometry (the science of continuous quantity). But isn’t life, and aren’t living things, all about qualitative diversity, and isn’t it the need for qualitative diversity that in turn calls for any quantitative diversity? Here I mean different organs made up of different matter, as it were: liver tissue being different from heart tissue, and heart tissue being different from lung tissue? Now, since liver tissue and heart tissue cannot be the same thing (and therefore cannot be in the same place), then it will be necessary that liver tissue be in a different location—i.e., be other than—heart tissue. So naturally there will be cell partitioning in the early embryo that produces different types of tissue; the cell cleavage that takes place in the zygote or thereafter in the early embryo is nothing other than the beginning of this tissue differentiation, without which the embryo will never be able to live outside of the womb—or even within the womb, since the embryo must produce the nutrition-gathering structures needed for its ongoing activity (i.e., the placenta). Cell division then (a bad way to label it: “cell-partitioning” is always better) is therefore not a sign of decreasing unity in a once-intact organism; it is rather the embryo’s effort to protect and further a functional unity that is needed for any living thing, especially one destined for extrauterine existence.
My sense is that the philosophers whom you are engaging do not start their thinking in the wet, sticky, smelly, busy world of living things. Rather their world begins in the tidy pantry of the mind, replete with ideas, definitions, theses, and logical entailments, which do not tolerate change. How can that latter world adequately address the constant change in a developing zygote? Any change in the zygote they will interpret as a fundamental loss of unity; one becomes two, the one has ceased, the zygote dies.
In truth it is they who kill the zygote, because their philosophical method never allows it to be alive in the first place.