That was a wonderful read. It's refreshing to run into an intelligent debate here. I agree we are both coming from a scientific perspective.
The fact that it survives separate from the mother but dependent on her furthers this in the same way that Siamese twins are separate people but have identical DNA.
Except Siamese twins are not separate people. They literally are not separable. They are conjoined people. Interesting term, don't you think? Not-separate plural people. Why is that? Because nearly every human culture values the mind above all else. If a person had one head but multiple pairs of legs, we'd say it was one person. Two heads means two people because the mind is what we value. As do zombies. It's why this society has no problems slaughtering animals for food.
Nevertheless, this gets into rather murky ethical territory that most people on this forum can't even begin to comprehend, but I'm reluctantly willing to discuss with you despite onlookers: why do we value babies or fetuses who do not yet possess such consciousness? Forget law, let's stick to ethics. I'll come back to science shortly.
I'll remove us from the topic slightly with a thought experiment. Say there are two dogs of the same age and breed facing certain doom, and only one could be saved. Now one of the dogs belongs to a family that loves it, and the other is a stray. Even though neither dog possesses consciousness, the decision of which to save is blatantly clear. That reasoning, combined with the potential for consciousness, is why we protect babies in this society. More notable is the fact that such potential for consciousness is held valuable to vastly different degrees for each person, which is ultimately the underlying issue being debated in every abortion discussion.
Firstly, let me make clear that I have made a major booboo.

When I stated ‘Siamese’ twins I did not actually mean Siamese twins. I realized reading your post that it is specific to conjoined twins. I was actually referring to identical twins, not conjoined twins as that brings this into another area of debate. Not that I am not happy to go there but I don’t think that my last post makes as much sense when viewed in that regard as I was putting that statement in there so that I was preemptively striking the idea that same DNA = same person. That was part of my initial thesis but I have to include separate bodies as well to deal with the identical twins aspect. Siamese twins on the other hand brings some other interesting points though as to what ‘separate’ entities entails. I don’t think that we need to get to that point though because an embryo clearly does not replicate this concept as it does not share the same body as the mother.
Now, as to the morality of the subject, I think you hit a correct concept but that it is somewhat unfinished. There is a clear decision in that because the idea of the one that is ‘loved’ where the other has none and we do regularly ‘euthanize’ animals that have no one that cares about them. The only thing I would add though is that we are programmed through biology to care for a child even when they are not ours. I do agree with your point though that the abortion debate does center on that ‘love’ aspect for children as well as the
potential development into fully away and cute babies and is a core reason that we tend to argue vehemently for protections of children. I can say for my part, there is nothing on this planet, nothing, that is more valuable than my two children and looking at them it is hard to accept the death of other children. Taken outside of the abortion debate, Sandy Hook is a good example where I would barely bat an eye at the death of 30 adults (hundreds die every day and there is nothing that I can do about it) but the death of so many children was heart wrenching.
OK so let's return to the science:
To understand this line of logic better, what would you consider a bacteria that lives within a human hosts’ stomach? That is clearly not ‘part’ of the host. It is a parasite (and I must point out that no matter how much emotion that word might bring into this debate - parasite is the most accurate way to describe a fetus). That parasite is unique in its existence. It is clearly a singular organism that is not a human. Its designation as a bacterium of a specific kind is found in its DNA, its individual status is found in the fact that it operates individually.
A fetus fits all the same criteria that a bacterium does. Why then are we so hesitant to ascribe it the same exact status as that bacterium.
Excellent point, but I would disagree by bringing up symbiosis here. Bacteria serve purpose in our intestines. We can survive without them, but we do better with them. If we want to see the ultimate one-cell organism symbiosis, we need but look at our own cells. Mitochondria, the cellular organelle that provides energy found in every cell, has completely different, non-human DNA. It divides separately of the greater cell in which it resides, and is in every way a ghost of what was once a completely separate organism. But now it's not separate. It cannot be removed without both counterparts dying.
So let's return to gut bacteria. Are they separate or separable? Yes, they can be removed without dying. Do they serve a purpose to the host? The good kind, sure. Are Siamese twins separate or separable? Is a fetus? If the answer is no, it does not fit the same criteria as the bacteria. Does it more closely approximate mitochondria?
That might be the ultimate example but it is irrelevant what example is used. Either way, that organism is its own organism and distinctly
not human. I will admit that I am at the end of my understanding of biology here as I am no biologist but, as I understand it, mitochondria ARE separate autonomous organisms that have simply lost the ability to survive without the host:
Endosymbiont - Wikipedia, the free encyclopedia
In that respect, that is exactly what a fetus is and neither is different than a bacterium. All three are entirely autonomous and operate under their own directive and own process though they respond to external stimuli. The fetus is close to the bacteria than mitochondria though as the host is not reliant on either and is actually harmed by the fetus through the removal of nutrients. Further, if you could sustain the nutrient flow and temperature, the fetus would indeed survive without the host.
Either way, I believe that it would be considered a separate organism.
BTW, you are making my brain hurt. I have to actually think about this
Lastly, I acknowledge that it is one more step in the developmental process. What I reject is that concept that developmental process starts with something other than a unique individual human. I am somewhat confused as to how you can state that it starts any other way. We do not start as a rock, lizard or another mammal. We start as a human, progress through the various developmental stages as a human and then die as a human. At no point along that entire chain are you not human nor at any point in time are you not a unique individual.
I disagree here, mostly because I don't see human cells and human beings as the same.
Nor do I but if you have just one cell then that is a person (at least to me) as any other definition speaks to me as a philosophical matter and NOT a scientific one. If we were to define human then, what would your definition be?
That is going to be an interesting challenge if it is not the culmination of the parts that make up our bodies aka cells.
Cancer creates unique human cells, but clearly cancer is not a human being. Nor does the uniqueness of the DNA actually matter, as we would both admit normal twins as two separate individual human beings. But one cell, while human, is neither a person, nor unique. It has the potential to become unique, but as I mentioned, potential is what the debate is all about. At the one-cell stage, it expresses the exact same proteins and markers as every other human embryo. There is literally nothing actively unique about it.
Except its autonomy and DNA (at least that of the embryo). Just because it is similar to other embryos does not make it an individual human. On that scale, we would all look alike to an alien race with extremely similar makeups. That does not mean we would cease to be individuals. Your statements ‘beg the question’ (oh yes, I went there

) at what point does it become a ‘human’ then. One cell is not enough but is 100, 100K, 10^100000? Where is that line? If it is other things than what exactly is it that you must have before you are considered ‘human.’
I see your stance on the matter, but I believe here is where we disagree. I see the human being part as something that is developed over time, without clear biologic borders.
So in summary: one cell not unique, uniqueness actually irrelevant, fetus not separate individual, embryos human genetically but not yet human beings.
I would call that ‘person.’
I realize that no one likes an argument in semantics (lest of all me) and I don’t want to get there BUT I want to point out why I make this delineation. Human, to me, is a very scientific term with a hard biological definition where the ‘person’ is culmination of the mind. As you hinted at before, we have different values on people and that value is very much tied to the mind. A person that is technically brain dead has no real value to anyone but the family and even then, only as long as it takes to say goodbye. After that, we usually disconnect them from the machines that are keeping them alive and let the body simply die.
This is because the ‘person’ is no longer there. Whoever you loved or knew is no longer present and what remains is only a shell. That body is still quite clearly ‘human,’ at least as I see it. It is not, however, a ‘person.’ Essentially, person is a subjective and soft term that is philosophical where human is not philosophical.