That's an interesting question.....
I think that the answer lies in the fact that there is no hope for the decapitated body. Those cells which make up a decapitated body are destined to die and fairly quickly.
A zygote on the other hand has the potential to become a human being. Not only that but it is constantly generating itself towards a point where it will take on the form of a human embryo. A zygote is in the process of generating life. A decapitated body is in the process of death and decomposing.
At the point where the zygote becomes an embryo and the brain begins to form, a new human being exists with a beating heart.
Your analogy, therefore, does not justify abortion since most abortions are carried out after six weeks gestation when the embryo already has a brain and a functioning heart.
[Note: Marie Stopes refuse to do abortions before six weeks gestation because they "cannot see whether it has been effective on the ultrasound scan".

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