Is a fertilized human cell a person?

I had an heretical thought the other day (heretical from the Pro-Life standpoint). The Bible claims in Lev. 17:11ff that "the life of a creature is in the blood".

Could that imply that a fertilized human cell does not become a living being until a blood cell forms in the growing embryo?

On the one hand, I'm well aware that from the moment of conception, nothing new is added to the growing embryo except nutrition.

On the other hand, sometimes in the early stages of embryonic development, the cell mass can divide and become identical twins (or triplets, etc). So, is the single fertilized egg one individual, or several?

If what makes a person a person is a human spirit, then:

1. there were two (or more) human spirits in the original single fertilized cell,

or

2. the spirit is being divided when the cell is divided into multiples

or

3. a second (or third, etc) spirit is being added to the newly-cleaved second (or third, etc) individual(s)

or

4. the spirit does not enter the embryo until sometime after differentiation begins.

If, on the other hand, what makes a person a person is the blood, then a fertilized egg is not a person until later in the developmental process.

I see two significant conclusions in such a case:

1) Very early abortions (within hours, not days) would not be murder.

2) Embryonic stem-cell research would not be murder.

I believe we should err on the side of caution, and on the side of Life, and not jump to these conclusions; my logic might be faulty, and the data is certainly sketchy.

It's of note that the embryo ceases to be divisible into twins long before the first blood cell develops. This seems a fatal objection to my idea above, and is further reason why we should err on the side of caution and treat a fertilized human cell as a person from conception onward.

Another consideration is that a person can be clinically dead while still having blood circulated by machines. Does that circulating blood keep the body alive without a human inhabiting it? If so, then the blood is not the determining factor of a person's life. Or perhaps the human is still in the body, and we just can't perceive that person any longer.

In short, I have no answers. But I found the thought interesting.