Tantalus Prime and I have been discussing abortion. Tantalus takes exception to my observation that human life begins at conception. He believes that the humanity of a human zygote/embryo/fetus isn’t a scientific fact, but merely a matter of linguistics:

The exact moment at which a fertilizing egg becomes human… is a horrible scientific question! Is asking the exact size beyond which a stream becomes a river a good scientific question? Of course not, because this is not an empirical problem but one of definition. Define the term human however you want, but don’t pretend it is an empirical question to be solved. Scientists can’t even agree on what constitutes a living organism, so what makes you think pinpointing the demarcation between human being and not human being is easily solved in a testable and falsifiable manner….

Tantalus doesn’t like to mix definitions with his science. Especially definitions that aren’t congenial to his ideology.
Yet a human embryo is surely something. But I’m not going to ask Tantalus that nasty beginning of life question. I’ll ask Tantalus a different question, hopefully one that he finds less irritating:
What is a human embryo?
There would seem to be 5 different things that a human embryo in the womb could be:
1) The human embryo is part of the mother.
2) The human embryo is not part of the mother, and is of a non-human species (i.e. not Homo sapiens).
3) The human embryo is some hitherto unclassified thing, neither of any species nor a part of the mother.
4) The human embryo is a proto-human being.
5) The human embryo is a human being.
Let’s explore Tantalus’ dilemma:

1) The human embryo is part of the mother. Tantalus asserts that women reproduce by a form of… budding. Budding is the emergence of a new organism from a part of the parent organism. At some point (don’t ask when) the ‘baby’–a new Homo sapiens— emerges from the human embryo, which prior to the emergence was a part of the mother’s body. Budding is an asexual method of reproduction in some unicellular organisms and metazoans, but Tantalus’ discovery of budding in female humans will revolutionize biology. Of course, male budding (the budding of Athena from the head of Zeus) was documented by the Greeks a few millennia ago, but the observation that all human beings arise by female budding places Tantalus in the first rank of scientists, superseded only by the ancient Greek dramatists.
2) The human embryo is not part of the mother, and is of a non-human species (i.e. not Homo sapiens) This assertion, if confirmed, should win Tantalus two Nobel Prizes. First, Tantalus has discovered the most widespread parasitic disease known to humanity–pregnancy. Second, Tantalus has shown that one species–Homo embryo–evolves over a period of nine months or so into a new species–Homo sapiens. And it happens like billions of times! Finally, we have observed… the Origin of Species! Homo embryo speciates into Homo sapiens with each pregnancy. Tantalus has finally proven Darwin’s theory true! It was right under our noses (actually navels) all this time.
3) The human embryo is some hitherto unclassified thing, neither of any species nor a part of the mother. If Tantalus is right, this is a discovery that will shake the foundations of biology. The human embryo is a living thing intrinsically outside of taxonomy. And what’s even more remarkable is that… what ever the heck it is, it becomes a human being if the mother host decides to let it live! We could call it Homo abortus. But then it would be… a species within taxonomy, in which case, see #2 above.
4) The embryo is a proto-human being. A meaningless assertion. If a “proto-human being” is not a human being (which is implied by “proto-“), then see # 1, #2, or #3 above. If If a “proto-human being” is a human being (i.e. a human being in its earliest stages), see #5 below.
5) The human embryo is a human being. This is the answer that Tantalus finds preposterous.
So Tantalus, what is a human embryo?