> the conceived egg cell is already a new member of the given species. why? has it to be of a species? if yes, which one other than human?
A cell being of a species does not make it an instance of that species, in the same way that a skin cell is a human cell but is not itself a human.
> "Can a human be a single cell?" can it be 2? or 3? …
If a single cell is not an instance of a human, it follows that there is some point during pregnancy where an instance of a human exists where one did not previously. Mostly the disagreement is about where this line is.
> in the same way that a skin cell is a human cell but is not itself a human.
IMO, there is a substantial difference between a conceived egg and any other cells: an egg turns into a human being over time (provided it's left doing its businnes normally), other kind of cells don't operate this way AFAIK.
> there is some point during pregnancy where an instance of a human exists where one did not previously
completely agree. IMO we can even extend it by omitting the "If a single cell is not an instance of a human" condition: even if we qualify a specific single cell to be a human, there must be a point when it became a human.
A cell being of a species does not make it an instance of that species, in the same way that a skin cell is a human cell but is not itself a human.
> "Can a human be a single cell?" can it be 2? or 3? …
If a single cell is not an instance of a human, it follows that there is some point during pregnancy where an instance of a human exists where one did not previously. Mostly the disagreement is about where this line is.